[{"data":1,"prerenderedAt":923},["ShallowReactive",2],{"layer:body-systems:investigate":3},{"layer":4,"contentHash":905,"dependencyHashes":906,"approval":917,"releaseId":922},{"schemaVersion":5,"conceptId":6,"locale":7,"depth":8,"revision":5,"title":9,"subtitle":10,"summary":11,"objectives":12,"estimatedMinutes":18,"plate":19,"blocks":40,"sourceIds":900,"reviewStatus":901,"authoring":902},1,"body-systems","en","investigate","Predict it, then test it","Reaction time, a real enzyme test, exercise data and a fever that is not a malfunction","Turn the claims from earlier layers into experiments you can actually run: a ruler-drop reaction test, an iodine test for digested starch, pulse and breathing data before and after exercise, and a look at why a fever is a controlled response rather than a failure.",[13,14,15,16,17],"Explain why muscles work in opposing pairs, and find the one hand-over that carries force rather than a substance.","Use a falling ruler and the physics of free fall to measure and calculate a real reaction time.","Design and explain a fair test with a control, using the amylase-and-iodine experiment as the model.","Compare how much pulse and breathing rate rise with exercise, using real data.","Explain fever as a deliberately raised set point, not a broken control system.",40,{"title":20,"rows":21},"Lesson plate",[22,25,28,31,34,37],{"label":23,"value":24},"Depth","Investigate",{"label":26,"value":27},"Reading time","≈ 40 minutes",{"label":29,"value":30},"Prior knowledge","Understand: mechanisms and hand-overs",{"label":32,"value":33},"Chapters","10",{"label":35,"value":36},"Labs","6: muscle flow, ruler, myth sort, breath, sizes, refill",{"label":38,"value":39},"Big idea","Predict, then test — and know a method's limits",[41,45,51,57,60,105,123,128,134,157,162,165,200,217,230,234,239,244,247,272,277,290,341,346,349,362,398,402,407,410,423,427,479,484,487,492,525,530,533,569,581,594,607,612,615,619,632,653,658,661,680,684,689,692,720,724,749,871,874,891],{"id":42,"type":43,"markdown":44},"intro-investigate","prose","Discover introduced the seven systems. Understand explained how each hand-over works. This layer asks you to stop taking the book's word for it: **predict what will happen, then actually test it.**\n\nReal science works this way. You state, in advance, what you expect — out loud or on paper, so you cannot quietly move the goalposts afterwards. Then you run the test. Sometimes you are right, and that is satisfying. Sometimes you are wrong, and that is more useful, because now you know precisely where your idea broke.\n\nEvery experiment below uses only things found in an ordinary classroom or home: a ruler, a stopwatch, a mirror, your own pulse, a spoonful of cooked rice. None of it requires a laboratory, and none of it is dangerous if you follow the few safety notes.",{"id":46,"type":47,"variant":48,"title":49,"markdown":50},"careful-investigate-safety","callout","careful","Ground rules for every experiment here","Three rules cover everything in this layer:\n\n1. **Nothing here should hurt or embarrass anyone.** If a test involves a partner, both people agree first, and either can stop at any time.\n2. **Stop if you feel dizzy, unwell or out of breath in a way that worries you**, and tell an adult.\n3. **Write your prediction down before you test it.** A prediction you make up afterwards to fit the result is not a prediction — it is a guess dressed up as one.",{"id":52,"type":53,"title":54,"eyebrow":55,"navLabel":56},"ch1","chapter","Muscles: the last hand-over before movement","Chapter 01","1 Muscle flow",{"id":58,"type":43,"markdown":59},"muscle-mechanism","You met the nerve-to-muscle hand-over in Understand: a nerve ending releases a chemical, the muscle fibre receives it, and the fibre contracts. Here is what \"contracts\" is actually built from.\n\nInside a muscle fibre are two kinds of protein filament, laid out in overlapping rows. When the chemical signal arrives, calcium is released inside the fibre, and that calcium is the trigger that lets the two kinds of filament grip each other and **slide past one another**, like two combs pushed together. Thousands of fibres doing this at once is what you feel as the whole muscle shortening.\n\nNotice what is *not* happening: no single filament gets shorter. The filaments themselves stay the same length. The shortening is entirely down to how far they slide over each other — a small slide, repeated across thousands of fibres, adds up to a muscle that can shorten by a third of its length in a fraction of a second.",{"id":61,"type":62,"component":63,"componentVersion":5,"config":64,"objective":103,"textAlternative":104},"lab-flow-muscular","interactive","system-flow",{"system":65,"steps":66,"quiz":101,"handoverWith":102},"muscular",[67,72,77,81,86,91,96],{"id":68,"label":69,"text":70,"organ":71},"signal","Order arrives","The chemical signal from the motor nerve reaches the muscle fibre at the nerve-muscle junction.","motor end plate",{"id":73,"label":74,"text":75,"organ":76},"calcium","Calcium is released","Inside the fibre, the signal triggers a flood of calcium, which is the actual switch that starts the contraction.","muscle fibre",{"id":78,"label":79,"text":80,"organ":76},"slide","Filaments slide","Two kinds of protein filament grip and slide past each other. Neither filament gets shorter; they simply overlap more.",{"id":82,"label":83,"text":84,"organ":85},"whole","The whole muscle shortens","Thousands of fibres sliding together shorten the muscle by up to about a third of its resting length.","biceps",{"id":87,"label":88,"text":89,"organ":90},"pull","The tendon pulls","The shortened muscle pulls on the tendon it is anchored to, and the tendon transmits that pull to the bone.","tendon",{"id":92,"label":93,"text":94,"organ":95},"swing","The bone swings","The hand-over point. Force — not a substance — crosses from muscle to bone at the joint, and the limb moves.","humerus",{"id":97,"label":98,"text":99,"organ":100},"relax","Relax, and the partner pulls back","The muscle can only relax, never push. Straightening the limb again needs the opposing muscle to contract instead.","triceps",true,"skeletal","Follow one contraction from the nerve signal arriving to the bone actually swinging, and find the one hand-over that carries force instead of a substance.","Seven steps from a nerve signal to a moving limb, with the **skeletal system** drawn alongside.\n\nThe signal arrives at the junction, calcium floods the fibre, and two kinds of filament slide past each other — the whole mechanism of a contraction, repeated in thousands of fibres at once. The muscle shortens and pulls on its tendon.\n\nThe **hand-over** is the last step: the tendon transfers that pull to the bone at a joint. Every other hand-over in this topic carries a substance — oxygen, glucose, a chemical signal. This is the one that carries **force**, and it only goes one way: muscles pull bones, never the reverse.\n\nThe final step is a reminder that a muscle can only pull. To reverse the movement, a second muscle on the other side of the joint must contract instead. Turn the quiz on and it asks which step would fail if the calcium release did not happen (the fibre would receive the signal but never contract).",{"id":106,"type":107,"prompt":108,"options":109,"explanation":122},"predict-muscle-only-pull","prediction","A muscle can only pull, never push. If you cut every muscle in an arm except the biceps, leaving it perfectly healthy, what would happen when it contracted?",[110,113,116,119],{"id":111,"label":112},"a","The arm would bend and then straighten itself again",{"id":114,"label":115},"b","The arm would bend, and stay bent — nothing is left to straighten it",{"id":117,"label":118},"c","The arm would push straight out",{"id":120,"label":121},"d","Nothing; a muscle needs a partner to work at all","**The arm would bend, and stay bent.** The biceps can shorten and pull the forearm up perfectly well on its own — it does not need a partner to contract. What it needs a partner *for* is undoing what it just did.\n\nWith the triceps gone, nothing can pull the forearm back down again except gravity and the elbow's own limits. This is exactly why muscles work in **opposing pairs**: not because either one is too weak alone, but because a muscle is a one-way rope, and every joint needs at least two ropes pulling in opposite directions to move both ways.",{"id":124,"type":47,"variant":125,"title":126,"markdown":127},"try-feel-pairs","try_it","Feel an opposing pair in action","Hold your right forearm loosely with your left hand, gripping the front of the upper arm (the biceps) and the back of it (the triceps) at the same time.\n\nSlowly bend your elbow. You will feel the biceps bulge and go hard, while the triceps stays soft. Now slowly straighten it again: the triceps goes hard, and the biceps goes soft. At no point are both hard together — a resting muscle is soft, a working one is hard, and exactly one of the pair is working at a time for a slow, controlled movement.\n\nTry the same test on your calf (the muscle at the back) and your shin (the muscle at the front) while you point and then flex your foot.",{"id":129,"type":130,"conceptId":131,"relation":132,"explanation":133},"conn-anatomy-invest-muscle","connection","human-body-anatomy","related_to","The names and exact locations of the biceps, triceps, calf and shin muscles you just felt are set out properly in the anatomy topic.",{"id":135,"type":136,"itemId":137,"prompt":138,"check":139,"hints":151,"feedback":154},"p-invest-pairs","practice","body-systems.investigate-pairs","Why do muscles almost always work in opposing pairs rather than alone?",{"kind":140,"options":141,"correct":150},"choice",[142,144,146,148],{"id":111,"label":143},"A single muscle is too weak to move a joint",{"id":114,"label":145},"A muscle can only pull, so undoing a movement needs a second muscle pulling the other way",{"id":117,"label":147},"Pairs look more symmetrical",{"id":120,"label":149},"One muscle gets tired faster than two",[114],[152,153],"What can a muscle NOT do?","Think about what would happen if only one side of a joint had a muscle.",{"correct":155,"incorrect":156},"Correct. A muscle can shorten but never lengthen itself back out. Moving a joint both ways needs one muscle to bend it and a separate one, pulling from the other side, to straighten it again.","A muscle's whole vocabulary is \"pull\". To reverse a movement, something else must pull the opposite way — which is why muscles come in **opposing pairs**.",{"id":158,"type":53,"title":159,"eyebrow":160,"navLabel":161},"ch2","How fast is a decision, really?","Chapter 02","2 Reaction time",{"id":163,"type":43,"markdown":164},"reaction-time-intro","A **reflex** is decided in the spinal cord in about 20 ms. A **voluntary reaction** — seeing something and choosing to respond — takes roughly 200 ms, ten times longer, because it goes all the way to the brain and back. Can you actually measure that second number yourself, with nothing but a ruler?\n\nYou can. It uses the one piece of physics every falling object obeys: it speeds up at a fixed rate, about 9.8 metres per second every second, whatever it weighs. If you know how far something fell, you can work out how long it fell for — even if the \"something\" is a ruler and the \"how long\" is your own reaction time.",{"id":166,"type":62,"component":167,"componentVersion":5,"config":168,"objective":198,"textAlternative":199},"lab-ruler-drop","data-lab",{"datasets":169,"valueRange":186,"step":5,"challenges":189},[170],{"label":171,"values":172,"unit":185},"Ruler-drop test, one class of 12 (distance caught, cm)",[173,174,175,176,177,178,179,180,181,182,183,184],15,18,22,25,14,20,17,23,19,21,16,24,"cm",{"min":187,"max":188},5,35,[190,194],{"measure":191,"target":192,"prompt":193},"mean",19.5,"What is the mean catching distance for the class? (Answer: 19.5 cm.)",{"measure":195,"target":196,"prompt":197},"range",11,"How spread out are the results? Find the range. (Answer: 11 cm.)","Try the ruler-drop reaction test yourself, then plot a class's results and read off the mean, median and range.","**The test.** One person holds a ruler vertically, zero end down, between a partner's open thumb and finger without touching it. Without warning, they let go. The partner catches it as fast as they can, and you read off how many centimetres slipped through before it was caught.\n\n**The data.** A sample class of twelve got: 15, 18, 22, 25, 14, 20, 17, 23, 19, 21, 16, 24 cm. Mean **19.5 cm**, range **11 cm**.\n\nDrag the dots to enter your own class's results and the mean, median, mode and range recompute live. The challenges ask you to find each measure and to say, in your own words, what a big range would mean about a group of people (that reaction times vary a lot between individuals, and even for the same person from one try to the next).",{"id":201,"type":202,"title":203,"problem":204,"steps":205,"help":212},"we-ruler-to-time","worked_example","Turning a falling distance into a reaction time","The sample class's mean catching distance was **19.5 cm**. A falling object drops a distance d in time t according to d = ½ × g × t², where g ≈ 9.8 m\u002Fs². How long, in milliseconds, did it take the class on average to react?",[206,207,208,209,210,211],"Convert the distance to metres: 19.5 cm = 0.195 m.","Rearrange the formula for time: t = √(2d ÷ g).","Put in the numbers: t = √(2 × 0.195 ÷ 9.8) = √0.0398.","t ≈ 0.199 seconds, which is **199 milliseconds**.","Compare that with the 200 ms used earlier in this topic for a voluntary reaction to something **seen**. They agree closely — the ruler-drop test is, quite literally, measuring the same thing.","Now check the reflex claim. A knee-jerk reflex takes about 20 ms. In that time a dropped object falls only 0.2 cm — too small a gap to see or catch, which is exactly why doctors test reflexes with a tap on a tendon rather than a falling ruler.",{"simplerExplanation":213,"hints":214},"A ruler falls faster the longer it falls. Measuring how far it fell before being caught tells you, indirectly, how long the reaction took.",[215,216],"Divide the centimetres by 100 to get metres before using the formula.","Take the square root at the very end.",{"id":218,"type":107,"prompt":219,"options":220,"explanation":229},"predict-reflex-vs-voluntary","Could you use the same ruler-drop test to measure a **reflex** like the knee jerk, instead of a voluntary reaction?",[221,223,225,227],{"id":111,"label":222},"Yes, it would just give a smaller number",{"id":114,"label":224},"No — the fall in that time is only a couple of millimetres, far too small to read on a ruler",{"id":117,"label":226},"No, because reflexes do not involve any movement",{"id":120,"label":228},"Yes, but only if the ruler is heavier","**No — the fall is too small to measure this way.** In 20 ms a dropped object falls only about 0.2 cm, a couple of millimetres. No ruler marking is fine enough, and no human catching speed is precise enough, to read that off.\n\nThis is not a failure of the idea — it is a genuine limit of the method, and noticing it is good scientific thinking. Reflexes are tested a completely different way: tap a tendon and watch whether the muscle jerks, which needs no timing at all, only a working circuit.",{"id":231,"type":47,"variant":48,"title":232,"markdown":233},"careful-ruler-safety","Doing the ruler-drop test safely","Use a light plastic or wooden ruler, never anything sharp or heavy. Drop it from a modest height — about 30 cm above the catching hand is plenty. Never do this near anyone's face or eyes, and never throw the ruler; simply let it go straight down.",{"id":235,"type":130,"conceptId":236,"relation":237,"explanation":238},"conn-gravity-ruler","gravity","applied_in","The ruler-drop test only works because every falling object speeds up at the same fixed rate regardless of its mass — the free-fall rule explored fully in the gravity topic.",{"id":240,"type":53,"title":241,"eyebrow":242,"navLabel":243},"ch3","Watching an enzyme actually work","Chapter 03","3 Testing digestion",{"id":245,"type":43,"markdown":246},"test-amylase-intro","Understand told you that saliva contains amylase, an enzyme that starts cutting starch into sugar. That is a claim you can test directly, using a simple colour change that is one of the oldest tricks in a school laboratory.\n\n**Iodine solution turns blue-black in the presence of starch**, and stays its normal brown-orange colour if no starch is left. So the test is: take two samples of the same starchy food, treat one with saliva and one without, wait, then add iodine to both and compare.",{"id":248,"type":249,"title":250,"items":251},"steps-amylase-test","steps","The amylase test, step by step",[252,256,260,264,268],{"title":253,"tag":254,"text":255},"Prepare","two samples","Take two small, equal spoonfuls of cooled, cooked rice or a starch-and-water paste, in two separate cups.",{"title":257,"tag":258,"text":259},"Add saliva","to cup A only","Add a little saliva (your own, on a clean spoon) to cup A and stir. Leave cup B untouched as the control.",{"title":261,"tag":262,"text":263},"Wait","≈ 5 minutes","At room or body warmth, amylase needs a few minutes to do noticeable work — the same warmth your mouth stays at.",{"title":265,"tag":266,"text":267},"Add iodine","a drop in each","Add one drop of iodine solution to each cup, and look at the colour without stirring it in.",{"title":269,"tag":270,"text":271},"Compare","the result","Cup B (no saliva) should turn dark blue-black — starch is still there. Cup A (with saliva) should stay closer to its original colour, or turn a much paler blue — much of the starch has already been cut into sugar.",{"id":273,"type":47,"variant":274,"title":275,"markdown":276},"nuance-control-sample","nuance","Why cup B matters as much as cup A","It is tempting to skip cup B and just test the saliva sample. That would be a mistake, and here is why.\n\nSuppose cup A came out pale. Without cup B you cannot tell whether that is because the saliva worked, or because your rice never had much starch in it, or because your iodine solution was old and weak. Cup B — treated exactly the same way, minus the one thing you are testing — is your **control**. It tells you what \"no effect\" looks like, so that a difference between A and B can only be explained by the one thing that differed: the saliva.\n\nThis is the shape of a fair test in any science: change exactly one thing, keep everything else the same, and always keep a sample where you changed nothing to compare against.",{"id":278,"type":107,"prompt":279,"options":280,"explanation":289},"predict-cold-saliva","Suppose you repeated the test, but this time kept cup A (rice plus saliva) in a fridge instead of at room warmth. What would you expect?",[281,283,285,287],{"id":111,"label":282},"No difference — enzymes do not care about temperature",{"id":114,"label":284},"The starch would disappear even faster in the cold",{"id":117,"label":286},"The colour change would happen much more slowly, because cold enzymes work more slowly",{"id":120,"label":288},"The saliva would stop being an enzyme in the cold","**Much more slowly.** Enzymes are proteins with a particular shape, and that shape jiggles and works fastest at a particular temperature — for amylase, close to your own body temperature, around 37 °C. Cool it down and the molecules move more sluggishly, so the enzyme meets and cuts far fewer starch chains per minute. It has not been destroyed, only slowed.\n\nHeat it too far the other way — well above body temperature — and something different and permanent happens: the enzyme's shape is wrecked and it stops working for good. That is one reason very hot food can feel like it \"settles\" oddly, and why a fever, which raises body temperature by only a degree or two, does not damage your enzymes but a rolling boil would.",{"id":291,"type":62,"component":292,"componentVersion":5,"config":293,"objective":339,"textAlternative":340},"lab-sort-myths","sort-game",{"prompt":294,"bins":295,"items":305,"seconds":338},"Before you sort, guess for yourself: true, false, or \"it depends\"? Then read why.",[296,299,302],{"id":297,"label":298},"true","True",{"id":300,"label":301},"false","False",{"id":303,"label":304},"depends","It depends",[306,310,314,318,322,326,330,334],{"id":307,"label":308,"bin":300,"why":309},"gum","Chewing gum helps you digest food faster","Chewing gum adds no useful enzyme beyond ordinary saliva, and swallowed air from chewing can cause bloating. It does not speed digestion.",{"id":311,"label":312,"bin":300,"why":313},"knuckles","Cracking your knuckles causes arthritis","The pop is a gas bubble collapsing in the joint fluid. Long-term studies have found no link to arthritis, though repeated forceful cracking is not encouraged either.",{"id":315,"label":316,"bin":300,"why":317},"darklight","Reading in dim light ruins your eyesight permanently","It can cause temporary eye strain and tiredness, which goes away with rest — it does not cause lasting damage.",{"id":319,"label":320,"bin":300,"why":321},"coldair","Going out with wet hair gives you a cold","Colds are caused by viruses, not temperature. Cold, wet conditions can weaken some defences and viruses spread more indoors in winter, but the cold air itself is not the cause.",{"id":323,"label":324,"bin":297,"why":325},"chew32","Chewing your food more thoroughly helps digestion","Smaller pieces mean more surface for enzymes to act on — the paneer-cube arithmetic from Understand, in your own mouth.",{"id":327,"label":328,"bin":297,"why":329},"fullstomach","Exercising hard right after a big meal can feel unpleasant","Exercise sends blood to the muscles and away from the gut, right when digestion needs blood at the gut. The two demands compete.",{"id":331,"label":332,"bin":300,"why":333},"sneezeheart","Your heart stops when you sneeze","It briefly changes rhythm because of pressure changes in the chest, but it does not stop. A steady rhythm resumes immediately.",{"id":335,"label":336,"bin":300,"why":337},"growth","Cracking your bones or \"growing pains\" mean your bones are breaking","Growing pains are thought to be harmless muscle aches from a day of activity in growing legs, not damage to bone or muscle.",0,"Guess true, false or \"it depends\" for eight common claims about the body, then read the reasoning behind each.","Eight commonly heard claims, three bins: **True**, **False**, **It depends**.\n\nMost of the classic ones here are **false**: chewing gum does not speed digestion, cracking knuckles is not linked to arthritis, dim light does not permanently damage eyes, wet hair does not cause colds (viruses do), a sneeze does not stop your heart, and growing pains are not bone damage.\n\nTwo are genuinely **true** and both connect straight back to earlier layers: chewing more thoroughly really does help digestion (more surface for enzymes), and exercising hard on a full stomach really can feel unpleasant (blood is being asked to go to muscles and gut at the same time).\n\nThe point of the game is not memorising eight facts. It is noticing that a claim sounding plausible, or being widely repeated, is not evidence — the same lesson Understand taught with the tennis-court lung and the 10-to-1 bacteria ratio.",{"id":342,"type":53,"title":343,"eyebrow":344,"navLabel":345},"ch4","What running does to your breathing — measured","Chapter 04","4 Testing exercise",{"id":347,"type":43,"markdown":348},"breath-exercise-intro","Discover measured pulse before and after exercise. Do the same for **breathing rate**, and a genuine question appears: does breathing speed up by a bigger factor than the pulse does, or a smaller one? Predict first, then look at the numbers.",{"id":350,"type":107,"prompt":351,"options":352,"explanation":361},"predict-breath-vs-pulse","A class measures both pulse and breathing rate before and after one minute of star jumps. Which do you predict rises by the larger **factor** (how many times bigger, not how many beats or breaths bigger)?",[353,355,357,359],{"id":111,"label":354},"Pulse rises by the bigger factor",{"id":114,"label":356},"Breathing rate rises by the bigger factor",{"id":117,"label":358},"They rise by exactly the same factor",{"id":120,"label":360},"Neither changes much","Test it and see below — this is a genuinely close call, which is what makes it worth measuring rather than guessing.\n\nIn one real class's data, resting pulse rose from 77.3 to 134.2 bpm, a factor of **1.74**. Breathing rose from 20.5 to 38.8 breaths a minute, a factor of **1.89** — very slightly larger. Both roughly double, and both are driven by the same trigger you met in Discover: rising carbon dioxide in the blood, sensed by the brain.",{"id":363,"type":62,"component":167,"componentVersion":5,"config":364,"objective":396,"textAlternative":397},"lab-breath-data",{"datasets":365,"valueRange":383,"step":5,"challenges":386},[366,370],{"label":367,"values":368,"unit":369},"Breathing rate before exercise, per minute",[174,178,175,179,184,178,181,182,175,174,176,178],"breaths\u002Fmin",{"label":371,"values":372,"unit":369},"Breathing rate straight after exercise",[373,374,375,376,377,374,378,18,379,380,381,382],34,38,42,32,46,36,41,33,48,37,{"min":384,"max":385},10,55,[387,390,393],{"measure":191,"target":388,"prompt":389},20.5,"Find the mean resting breathing rate. (Answer: 20.5 breaths\u002Fmin.)",{"measure":191,"target":391,"prompt":392},38.8,"Find the mean breathing rate after exercise. (Answer: 38.8 breaths\u002Fmin.)",{"measure":394,"target":178,"prompt":395},"median","Find the median resting breathing rate. (Answer: 20 breaths\u002Fmin.)","Plot a class's breathing rate before and after exercise, and compare how much it rises with how much the pulse rises.","Two dot plots, editable, of the same twelve children's breathing rate: 18, 20, 22, 17, 24, 20, 19, 21, 22, 18, 25, 20 breaths a minute at rest, rising to 34, 38, 42, 32, 46, 38, 36, 40, 41, 33, 48, 37 straight after one minute of star jumps.\n\nMean rises from **20.5** to **38.8**, a factor of **1.89**. Set this beside the pulse data from Discover, where the mean rose by a factor of 1.74. The two are close, and both are driven by the same rising-carbon-dioxide alarm — the heart and the lungs are not two separate decisions, they are one decision acted out by two systems at once.\n\nThe challenges ask for the mean, median and a comparison between the two datasets' spread — is the class more alike or less alike once everyone is out of breath?",{"id":399,"type":130,"conceptId":400,"relation":237,"explanation":401},"conn-data-handling-breath","data-handling","Comparing how much two measurements rise by, as a factor rather than a raw difference, is exactly the kind of reasoning data handling gives you the tools for.",{"id":403,"type":53,"title":404,"eyebrow":405,"navLabel":406},"ch5","Fever: a controlled response, not a malfunction","Chapter 05","5 Fever",{"id":408,"type":43,"markdown":409},"fever-intro","Everything in Understand's homeostasis chapter described the body defending a **fixed** target, 37.0 °C. A fever looks, at first, like that system failing — temperature climbing when it is supposed to stay put. It is the opposite. In a fever, the body's control centre deliberately **raises its own target**, usually because chemicals released by the immune system tell it to.\n\nMany germs grow best at the body's normal temperature and struggle a little above it. Raising the target a degree or two makes the whole body a slightly worse home for them, while your own cells cope with the change easily. A fever, within reason, is your immune system turning up the thermostat on purpose.",{"id":411,"type":107,"prompt":412,"options":413,"explanation":422},"predict-fever-chills","Right at the start of a fever, before the temperature has actually risen, a person often feels **cold and shivers**, even though a thermometer would show their temperature is normal or only just starting to climb. Why?",[414,416,418,420],{"id":111,"label":415},"The thermometer is wrong",{"id":114,"label":417},"Their control centre has already raised its target, so their current, still-normal temperature now feels too low to it",{"id":117,"label":419},"Fevers always start with the temperature falling first",{"id":120,"label":421},"Shivering causes the fever, not the other way round","**Their target has already jumped, before their actual temperature has caught up.** Imagine a room thermostat set to 20 °C suddenly reset to 24 °C. The room is still at 20 °C — perfectly comfortable a moment ago — but the heater now reads that as \"too cold\" and switches on. You would feel a draught that was not there before, even though nothing in the room itself changed.\n\nThat is exactly a chill at the start of a fever: shivering and feeling cold are the body chasing its **new, higher** target. Once body temperature has climbed to match it, the shivering stops and the chilled feeling passes — the same control loop, just aimed at a different number for a while.",{"id":424,"type":47,"variant":274,"title":425,"markdown":426},"nuance-fever-calm","A calm word about fevers","A mild fever fighting off an ordinary infection is not automatically dangerous, and treating every small rise as an emergency misses what it is actually doing. It is also true that a high or long-lasting fever, or one in a very young child, needs an adult and sometimes a doctor — the body's thermostat is a clever tool, not an unlimited one.\n\nThe useful habit is simple: rest, drink water, and tell an adult if a fever is high, lasts more than a couple of days, or comes with other worrying signs. This is one topic where \"ask an adult\" is the entire correct answer for a child, and there is no need to self-diagnose anything further.",{"id":428,"type":429,"title":430,"prompt":431,"options":432},"explorer-body-sizes","explorer","Guess the size, then check","Pick an organ and see how close your guess was.",[433,446,456,467],{"id":434,"label":435,"chain":436,"badge":442,"note":445},"heart","Heart",[437,438,439,440,441],"Guess a size","Compare to your fist","About right","Roughly 300 g","Beats on, unpaused",{"text":443,"tone":444},"About one fist","yes","Roughly the size of its owner's own clenched fist, and about the same in a small child as the child's own small fist — the heart grows along with the rest of the body. Most people guess bigger.",{"id":447,"label":448,"chain":449,"badge":453,"note":455},"kidney","Kidney",[437,438,450,451,452],"Close, slightly smaller","About 150 g each","Two, working as a pair",{"text":454,"tone":444},"A little smaller than a fist","Each kidney is a little smaller than a clenched fist, roughly the size and shape of a large bean (which is exactly where the shape \"kidney bean\" gets its name). Two together weigh under a third of a kilogram, yet handle the enormous filtering job from Understand.",{"id":457,"label":458,"chain":459,"badge":464,"note":466},"liver","Liver",[437,460,461,462,463],"Compare to a football","Bigger than expected","About 1.5 kg","The heaviest internal organ",{"text":465,"tone":444},"About a small football","The largest internal organ, roughly the size and weight of a small football and easily the heaviest organ inside the body — most people guess far too small, because it sits quietly behind the ribs on the right and is never felt directly the way a full stomach is.",{"id":468,"label":469,"chain":470,"badge":476,"note":478},"smallgut","Small intestine (in life)",[471,472,473,474,475],"Guess a length","Compare to the room","Longer than guessed","About 3 m in a living body","Kept in constant muscle tone",{"text":477,"tone":444},"About 3 metres alive","In a living body, held in constant muscle tone, the small intestine is about 3 metres long — roughly the height of two adults. (It relaxes to nearer 6–7 metres after death, which is the figure many older books quote, causing genuine confusion between sources.)",{"id":480,"type":53,"title":481,"eyebrow":482,"navLabel":483},"ch6","Same design, different people","Chapter 06","6 Individual differences",{"id":485,"type":43,"markdown":486},"individual-diff","Every number in this topic — resting pulse, breathing rate, reaction time, sleep needed — has been given as a typical range, not a single fixed value, and that was deliberate. Real bodies vary, healthily, for reasons that include age, fitness, body size, genetics and simply being a different person.\n\nThis is worth investigating directly rather than assuming. Two people of the same age can have resting pulses ten or twenty beats apart and both be perfectly healthy. A fair test compares a **person with themselves** — before and after exercise, or one week and the next — far more reliably than it compares one person with another.",{"id":488,"type":47,"variant":489,"title":490,"markdown":491},"model-limit-averages","model_limit","What a class average hides","The ruler-drop mean of 19.5 cm and the pulse mean of 77.3 bpm are useful summaries, but a mean describes a **group**, not any one member of it. Look back at the ruler-drop range: 11 cm separated the fastest and slowest reactor in a class of just twelve.\n\nBe careful never to read a class average as \"the normal amount for a person of your age\" and worry if you land above or below it. Ranges exist precisely because normal is wide. The interesting scientific question is almost never \"am I average?\" — it is \"does my own measurement change the way the theory predicts it should?\"",{"id":493,"type":62,"component":494,"componentVersion":5,"config":495,"objective":523,"textAlternative":524},"lab-match-experiments","match-pairs",{"prompt":496,"mode":497,"pairs":498},"Match each experiment to what it actually measures or shows.","connect",[499,502,505,508,511,514,517,520],{"a":500,"b":501},"Ruler-drop test","Voluntary reaction time",{"a":503,"b":504},"Tendon-hammer tap (knee jerk)","Reflex speed, decided in the spinal cord",{"a":506,"b":507},"Iodine added to rice plus saliva","Whether starch has been digested into sugar",{"a":509,"b":510},"Pulse before and after star jumps","How much the heart rate rises with exertion",{"a":512,"b":513},"Breathing count before and after exercise","How much breathing rate rises with exertion",{"a":515,"b":516},"Chewing roti for one minute","Amylase turning starch into sugar, tasted as sweetness",{"a":518,"b":519},"Feeling your calf while walking","Muscles squeezing veins to help blood return to the heart",{"a":521,"b":522},"Holding your breath and timing it","How carbon dioxide, not oxygen, triggers the urge to breathe","Connect eight simple experiments from this topic with the thing each one actually demonstrates.","Eight everyday tests, matched to what each one is really evidence for.\n\n- **Ruler-drop** ↔ voluntary reaction time (via the physics of falling).\n- **Tendon-hammer tap** ↔ a spinal reflex, far faster than a voluntary reaction.\n- **Iodine on rice plus saliva** ↔ whether starch has already been cut into sugar.\n- **Pulse before\u002Fafter exercise** ↔ how much harder the heart works under load.\n- **Breathing count before\u002Fafter exercise** ↔ the matching rise in breathing.\n- **Chewing roti slowly** ↔ amylase working, noticed as sweetness.\n- **Feeling your calf while walking** ↔ the \"second heart\" effect of muscles squeezing veins.\n- **Timed breath-holding** ↔ carbon dioxide, not a shortage of oxygen, driving the urge to breathe.\n\nNone of these need special equipment. That is the point of an Investigate layer: the claims in this topic are not mysterious facts to accept, they are things you can go and check.",{"id":526,"type":53,"title":527,"eyebrow":528,"navLabel":529},"ch7","How fast do you recover? A second measurement","Chapter 07","7 Recovery time",{"id":531,"type":43,"markdown":532},"recovery-intro","Discover's \"find your pulse\" activity asked you to keep counting once a minute until you were back to your starting rate, and to write down how long that took. That single number — recovery time — turns out to be one of the more informative measurements in this whole topic, and it is not the same as your resting pulse or your peak pulse.\n\nTwo people can reach the exact same peak pulse after the exact same effort, and still recover at very different speeds. The one who calms back down faster is, on this particular measure, fitter — regardless of what their resting pulse was to start with.",{"id":534,"type":535,"caption":536,"columns":537,"rows":541},"table-recovery","table","One-minute pulse readings after identical hard exercise, two fitness levels.",[538,539,540],"Minutes after stopping","Fitter person (bpm)","Less fit person (bpm)",[542,545,549,553,557,561,565],[543,544,544],"0","134",[546,547,548],"1","118","126",[550,551,552],"2","104","119",[554,555,556],"3","96","113",[558,559,560],"4","90","108",[562,563,564],"5","84","103",[566,567,568],"6","79","99",{"id":570,"type":202,"title":571,"problem":572,"steps":573,"help":579},"we-recovery-ratio","Comparing two recovery curves","Both people reach the same peak, 134 bpm, after the same hard effort. In the first minute afterwards, the fitter person's pulse drops to 118 bpm and the less fit person's drops to 126 bpm. Compare how much each pulse actually fell.",[574,575,576,577,578],"Fitter person's drop: 134 − 118 = **16 bpm** in one minute.","Less fit person's drop: 134 − 126 = **8 bpm** in one minute.","Ratio: 16 ÷ 8 = **2**.","The fitter heart is not just faster to slow down — in this first minute it recovers **twice** as fast, from an identical starting point and an identical effort.","This is exactly why doctors and sports coaches use recovery time, not resting pulse alone, as a fitness signal: it isolates how quickly the system returns to normal once the demand stops, rather than where it happened to start.",{"simplerExplanation":580},"Subtract to find how many beats each pulse fell in the first minute, then divide the two drops to compare them.",{"id":582,"type":136,"itemId":583,"prompt":584,"check":585,"hints":589,"feedback":591},"p-invest-recovery","body-systems.investigate-recovery","Using the table above, by how many beats per minute does the less fit person's pulse fall between minute 1 and minute 2 (126 to 119 bpm)?",{"kind":586,"answer":587,"tolerance":338,"unit":588},"number",7,"bpm",[590],"Subtract the minute-2 reading from the minute-1 reading.",{"correct":592,"incorrect":593},"Right: 126 − 119 = **7 bpm**. Compare this with the fitter person's drop over the same minute and notice the gap is narrowing — both curves flatten out as they approach a normal resting rate.","Take the two readings from the table for the less fit person at minute 1 and minute 2, and subtract.",{"id":595,"type":107,"prompt":596,"options":597,"explanation":606},"predict-recovery-meaning","Two classmates do the identical hard run and reach the identical peak pulse. One is back to their resting rate in about four minutes; the other takes twelve. Both feel fine afterwards. What is the fairest conclusion?",[598,600,602,604],{"id":111,"label":599},"The slower-recovering one is unwell and should see a doctor immediately",{"id":114,"label":601},"On this particular measure, the faster-recovering one is currently fitter — it says nothing else about either person",{"id":117,"label":603},"The faster-recovering one has a smaller heart",{"id":120,"label":605},"Recovery time is random and means nothing","**It is one genuine signal of current fitness, and nothing more dramatic than that.** Recovery time reflects how efficiently the whole system — heart, blood vessels and the nervous control switching them back to \"resting\" mode — settles down once a demand ends, and it improves with regular exercise over weeks, for almost anyone.\n\nIt is not a verdict on health, cleverness or worth, and a single day's measurement can be thrown off by heat, sleep, or simply how hard the run actually was for each person. Used sensibly — tracking your own recovery time over a school term, say — it is a genuinely useful, judgement-free number to watch.",{"id":608,"type":53,"title":609,"eyebrow":610,"navLabel":611},"ch8","Does food need gravity to go down?","Chapter 08","8 Peristalsis test",{"id":613,"type":43,"markdown":614},"peristalsis-test-intro","Discover's steps described peristalsis — a wave of muscle squeezing behind the food and relaxing in front of it — as working \"even upside down\". That is a specific, testable claim, and you do not need to actually hang upside down to test a safe version of it.",{"id":616,"type":47,"variant":125,"title":617,"markdown":618},"try-swallow-lying","Swallow lying flat, then swallow tilted back","With an adult's permission, try this with a small sip of water. First, sitting normally, take a sip and swallow — completely ordinary.\n\nNow lie flat on your back on a bed or the floor and take a small sip through a straw, then swallow. Notice that it still works, smoothly, even though gravity is now pulling the water sideways relative to your throat rather than straight down toward your stomach.\n\nIf you have ever watched footage of an astronaut eating and drinking in orbit, this is exactly why it works at all: peristalsis is muscle doing the pushing, not gravity doing the falling. Never try this with a large mouthful, and always sit back up slowly afterwards.",{"id":620,"type":107,"prompt":621,"options":622,"explanation":631},"predict-astronaut-swallow","An astronaut on the International Space Station, in continuous freefall with no effective gravity, eats a spoonful of food. Based on what peristalsis actually is, what happens?",[623,625,627,629],{"id":111,"label":624},"The food cannot go down at all without gravity",{"id":114,"label":626},"It travels down the oesophagus perfectly normally, squeezed along by peristalsis",{"id":117,"label":628},"It floats around inside the body",{"id":120,"label":630},"Astronauts can only drink through a vein in space","**It travels down normally.** Peristalsis is a wave of **muscle** contraction squeezing the tube itself, not a controlled fall relying on gravity. It works lying down, tilted, upside down, or in orbit with no effective gravity at all, because the mechanism never depended on gravity in the first place.\n\nThis is a genuinely good way to spot which body processes secretly rely on gravity and which do not. Peristalsis: no. Blood returning through leg veins on Earth: yes, partly (which is why the \"second heart\" calf-muscle-pump story in Understand matters so much on the ground, and why astronauts' fluid shifts headward in Extend's microgravity chapter — a process that does depend on gravity, floods differently once gravity is removed).",{"id":633,"type":136,"itemId":634,"prompt":635,"check":636,"hints":647,"feedback":650},"p-invest-peristalsis","body-systems.investigate-peristalsis","Why does peristalsis keep working even if you are lying down or upside down?",{"kind":140,"options":637,"correct":646},[638,640,642,644],{"id":111,"label":639},"Gravity still helps a little from any position",{"id":114,"label":641},"It is a wave of muscle contraction squeezing the tube, not a controlled fall relying on gravity",{"id":117,"label":643},"Saliva pulls the food down regardless of position",{"id":120,"label":645},"It only works for liquids, not solids",[114],[648,649],"What actually pushes food along the oesophagus?","Is peristalsis a fall, or a squeeze?",{"correct":651,"incorrect":652},"Correct. Peristalsis physically squeezes the food along the tube using muscle, so it works in any orientation — lying down, tilted, or in orbit.","Peristalsis is a muscular squeeze, not a fall under gravity, so it does not depend on which way is \"down\".",{"id":654,"type":53,"title":655,"eyebrow":656,"navLabel":657},"ch9","How fast does colour come back?","Chapter 09","9 Capillary refill",{"id":659,"type":43,"markdown":660},"capillary-refill-intro","Here is a test that needs no equipment at all beyond your own fingernail and a way to count seconds. Press firmly on a fingernail until the flesh underneath turns pale — you have just squeezed the blood out of the capillaries there. Let go, and time how long it takes the pink colour to return.\n\nThis is called **capillary refill**, and it is a genuinely quick, real signal of how well blood is being delivered to your extremities, used in exactly this simple form by doctors and first-aiders as a rough, immediate check.",{"id":662,"type":62,"component":167,"componentVersion":5,"config":663,"objective":678,"textAlternative":679},"lab-cap-refill",{"datasets":664,"valueRange":671,"step":5,"challenges":672},[665],{"label":666,"values":667,"unit":670},"Capillary refill time, one class of 12 (seconds)",[5,668,668,5,669,668,5,668,669,5,668,668],2,3,"s",{"min":338,"max":187},[673,676],{"measure":191,"target":674,"prompt":675},1.83,"Find the mean refill time for the class. (Answer: 1.83 s.)",{"measure":195,"target":668,"prompt":677},"Find the range. (Answer: 2 s.)","Test your own capillary refill time, then plot a class's results and compare with a commonly used rough guideline.","**The test.** Press a fingernail firmly for a few seconds until it turns pale, release, and count how many seconds it takes to return to its normal pink colour.\n\n**Sample data.** A class of twelve got: 1, 2, 2, 1, 3, 2, 1, 2, 3, 1, 2, 2 seconds. Mean **1.83 s**, range **2 s** — all comfortably inside the commonly used rough guideline of well under 2 seconds for a warm, healthy hand.\n\nThis is one more small, safe measurement you can genuinely repeat: cold hands, a hand held above the head versus hanging down, or straight after exercise are all interesting, harmless variations to test and compare against your own resting value.",{"id":681,"type":47,"variant":274,"title":682,"markdown":683},"nuance-refill-not-diagnosis","A useful signal, not a diagnosis","A slow capillary refill can mean several ordinary, harmless things — cold hands being the most common by far — as well as occasionally being a sign worth a doctor's attention if it stays slow along with other symptoms. A single reading, done for interest in a classroom, tells you nothing you should worry about on its own.\n\nThe value of this test for this topic is what it demonstrates, not what it diagnoses: it makes the circulatory system's constant, silent job of refilling every last capillary something you can watch happen, in real time, on your own hand.",{"id":685,"type":53,"title":686,"eyebrow":687,"navLabel":688},"ch10","Design your own test","Chapter 10","10 Design a test",{"id":690,"type":43,"markdown":691},"design-your-own","This layer has walked you through five ready-made experiments. The last and best test of whether you have understood the idea of a fair test is designing one yourself, from scratch, for a claim nobody has checked for you yet.",{"id":693,"type":249,"title":694,"items":695},"steps-design-checklist","A checklist for designing a fair test",[696,700,704,708,712,716],{"title":697,"tag":698,"text":699},"1. State the claim","clearly","Write the claim as a single sentence you could imagine being wrong. \"Chewing gum speeds digestion\" is testable; \"digestion is complicated\" is not.",{"title":701,"tag":702,"text":703},"2. Decide what to measure","a number or a clear yes\u002Fno","Pick something you can actually observe or count, like the ruler-drop distance or the iodine colour, not a vague feeling.",{"title":705,"tag":706,"text":707},"3. Change exactly one thing","the variable","Everything else — the food, the amount, the timing, the person — should be kept as similar as possible between your test and your control.",{"title":709,"tag":710,"text":711},"4. Keep a control","the \"nothing changed\" case","Without one, you cannot tell whether your result came from the thing you changed or from something else entirely.",{"title":713,"tag":714,"text":715},"5. Predict first","before you look","Write down what you expect to happen, so the result can genuinely surprise you.",{"title":717,"tag":718,"text":719},"6. Run it, and be honest","about the result","Report what actually happened, even if it does not match your prediction — that mismatch is often the most useful part.",{"id":721,"type":722,"prompt":723},"reflect-design-test","reflection","Choose one untested claim of your own about the body — something you have heard but never checked (a school-yard rumour, something a relative always says, or one of your own guesses about your own body).\n\nUsing the six-step checklist above, write a short experiment design: the claim, what you would measure, what you would keep the same, your control, your prediction, and what result would prove you wrong. You do not have to run it — designing it honestly is the point of this exercise.",{"id":725,"type":726,"title":727,"terms":728},"glossary-investigate","glossary","Words for this layer",[729,733,737,741,745],{"term":730,"meaning":731,"example":732},"Control (in an experiment)","The sample or condition left unchanged, so you can tell what \"no effect\" looks like.","Cup B, with no saliva added, in the amylase test.",{"term":734,"meaning":735,"example":736},"Fair test","Changing exactly one thing at a time while keeping everything else the same.","Same rice, same amount of iodine — the only difference is saliva or not.",{"term":738,"meaning":739,"example":740},"Mean, median, mode, range","Four different summaries of a set of measurements: the average, the middle value, the most common value, and the spread from lowest to highest.","A pulse data set with mean 77.3 bpm and range 20 bpm.",{"term":742,"meaning":743,"example":744},"Set point","The value a control system is currently trying to hold — usually fixed, but reset upward on purpose during a fever.","Body temperature's normal set point is about 37 °C.",{"term":746,"meaning":747,"example":748},"Opposing pair","Two muscles arranged so one bends a joint and the other straightens it, because a muscle can only pull.","Biceps and triceps.",{"id":750,"type":751,"title":752,"questions":753},"quiz-investigate","quiz","Check yourself: predicting and testing",[754,767,780,793,806,819,832,845,858],{"itemId":755,"prompt":756,"options":757,"correct":117,"why":766},"body-systems.investigate-q-pull","What can a single muscle, acting alone, never do?",[758,760,762,764],{"id":111,"label":759},"Contract",{"id":114,"label":761},"Pull a tendon",{"id":117,"label":763},"Lengthen itself back out again",{"id":120,"label":765},"Get tired","A muscle can only shorten (pull). Lengthening it back out needs an opposing muscle, or gravity, to do the work.",{"itemId":768,"prompt":769,"options":770,"correct":114,"why":779},"body-systems.investigate-q-ruler","The ruler-drop test can measure a voluntary reaction but not a knee-jerk reflex. Why not?",[771,773,775,777],{"id":111,"label":772},"Reflexes involve no movement",{"id":114,"label":774},"The fall in a reflex's time is only a couple of millimetres — too small to read",{"id":117,"label":776},"The ruler moves upward during a reflex",{"id":120,"label":778},"Reflexes are too fast for gravity to act at all","In about 20 ms an object falls only a couple of millimetres, far below what a ruler and a human catch can resolve. A different test — a tendon tap — is used instead.",{"itemId":781,"prompt":782,"options":783,"correct":114,"why":792},"body-systems.investigate-q-control","In the amylase test, what is cup B (rice with no saliva added) for?",[784,786,788,790],{"id":111,"label":785},"It is the important result",{"id":114,"label":787},"It shows what \"no digestion happened\" looks like, so cup A can be compared against it",{"id":117,"label":789},"It is a mistake and should be thrown away",{"id":120,"label":791},"It tests the iodine, not the rice","Cup B is the **control**. Without it, a pale colour in cup A could have several explanations; with it, the only difference left is the saliva.",{"itemId":794,"prompt":795,"options":796,"correct":114,"why":805},"body-systems.investigate-q-fever","What actually happens at the start of a fever?",[797,799,801,803],{"id":111,"label":798},"The body's temperature control breaks down",{"id":114,"label":800},"The control centre deliberately raises its target temperature",{"id":117,"label":802},"The thermometer becomes inaccurate",{"id":120,"label":804},"Sweating stops working","The set point is raised on purpose, usually signalled by the immune system, making the body a worse home for many germs. Shivering at the start is the body chasing its new, higher target.",{"itemId":807,"prompt":808,"options":809,"correct":117,"why":818},"body-systems.investigate-q-both-rise","A class measures pulse and breathing rate before and after exercise. What do the real numbers in this topic show?",[810,812,814,816],{"id":111,"label":811},"Only pulse rises",{"id":114,"label":813},"Only breathing rises",{"id":117,"label":815},"Both rise by a similar factor, roughly doubling",{"id":120,"label":817},"Neither changes reliably","Pulse rose by a factor of about 1.74 and breathing by about 1.89 in the sample data — both roughly double, driven by the same rising-carbon-dioxide signal.",{"itemId":820,"prompt":821,"options":822,"correct":114,"why":831},"body-systems.investigate-q-fair-test","What makes a test \"fair\" in the scientific sense?",[823,825,827,829],{"id":111,"label":824},"Everyone gets the same mark",{"id":114,"label":826},"Only one thing is changed at a time, with everything else kept the same",{"id":117,"label":828},"The result is checked by a teacher",{"id":120,"label":830},"It uses expensive equipment","A fair test isolates a single variable. Change saliva and nothing else, and any difference in the result can only be explained by the saliva.",{"itemId":833,"prompt":834,"options":835,"correct":114,"why":844},"body-systems.investigate-q-peristalsis-quiz","Why does swallowing still work when you are lying flat, or even upside down?",[836,838,840,842],{"id":111,"label":837},"Gravity redirects itself",{"id":114,"label":839},"Peristalsis is a muscular squeeze along the tube, not a fall relying on gravity",{"id":117,"label":841},"Saliva becomes stickier",{"id":120,"label":843},"It does not actually work upside down","Peristalsis physically squeezes food along, using muscle — it works in any orientation because it was never a controlled fall in the first place.",{"itemId":846,"prompt":847,"options":848,"correct":114,"why":857},"body-systems.investigate-q-refill-quiz","What does a capillary refill test actually demonstrate?",[849,851,853,855],{"id":111,"label":850},"How much oxygen is in the blood",{"id":114,"label":852},"How quickly blood is being delivered back into capillaries that were squeezed empty",{"id":117,"label":854},"How strong your fingernails are",{"id":120,"label":856},"Your resting pulse","Pressing a nail squeezes blood out of its capillaries; timing the colour's return shows how quickly the circulatory system refills them — a simple, visible sign of ordinary, healthy delivery.",{"itemId":859,"prompt":860,"options":861,"correct":117,"why":870},"body-systems.investigate-q-design-quiz","A classmate tests a claim by changing several things at once — the food, the amount, and the timing. What is wrong with this test?",[862,864,866,868],{"id":111,"label":863},"Nothing — more changes give a stronger result",{"id":114,"label":865},"It has no control group",{"id":117,"label":867},"If a difference appears, you cannot tell which of the several changes actually caused it",{"id":120,"label":869},"It needs more expensive equipment","Changing several things at once destroys the test's fairness: any result could be explained by any one of the changes, or a combination, and you cannot isolate a cause.",{"id":872,"type":722,"prompt":873},"reflect-investigate","Pick one claim from this topic that you have not personally tested — perhaps the sweetness of chewed roti, your own resting pulse, or the ruler-drop test.\n\nWrite a short plan for testing it: what you would measure, what you would keep the same (your control), what result would confirm the claim, and what result would make you doubt it. Then, if you can, actually try it.",{"id":875,"type":876,"title":877,"points":878},"cheat-investigate","summary","Cheat sheet",[879,880,881,882,883,884,885,886,887,888,889,890],"**Muscles only pull.** Movement both ways needs opposing pairs (biceps\u002Ftriceps); the muscle-to-bone hand-over carries force, not a substance.","**Reaction time, measured with a falling ruler:** distance fallen gives time via d = ½gt². A class mean of about 19.5 cm corresponds to roughly 199 ms — close to the 200 ms used elsewhere in this topic.","A reflex (≈20 ms) falls too little to measure with a ruler — that is a real limit of the method, not a failure of the idea.","**A fair test changes one thing and keeps a control** — cup B with no saliva is what makes cup A meaningful.","Chewing more thoroughly genuinely helps digestion; most other \"common knowledge\" claims tested in this layer (gum, knuckle-cracking, wet hair, dim light) do not hold up.","**Exercise roughly doubles both pulse and breathing** — factors of about 1.74 and 1.89 in the sample data — from the same carbon-dioxide trigger.","**Fever is a reset target, not a broken thermostat.** Chills at the start happen because the target jumped before the body caught up.","Averages describe groups, not individuals — a wide range is normal, and comparing yourself with yourself over time is more meaningful than comparing with a class average.","**Recovery time is its own fitness signal**, separate from resting or peak pulse: in the sample data the fitter person's pulse fell 2 times as fast in the first minute after stopping.","**Peristalsis works in any orientation** — it is a muscular squeeze, not a gravity-assisted fall, which is why swallowing works lying down or in orbit.","**Capillary refill** — how fast colour returns after a fingernail is pressed pale — is a real, simple circulatory check; a class average around 1.83 s is typical.","A well-designed test states one testable claim, changes exactly one variable, keeps a control, and predicts before looking at the result.",{"id":892,"type":893,"sourceIds":894},"sources-investigate","sources",[895,896,897,898,899],"body-systems-britannica-cardiovascular","body-systems-britannica-respiratory","body-systems-britannica-nervous","body-systems-wiki-homeostasis","body-systems-britannica-digestive",[895,896,897,898,899],"needs_review",{"generatedBy":903,"notes":904},"claude-code","Draft generated locally; pending owner review.","dbd4afe5ff19827bfac40fa10622c8befabda58428e2a3daf265e40894e57fc6",{"component:system-flow@1":907,"logic:practice":908,"component:data-lab@1":909,"component:sort-game@1":910,"component:match-pairs@1":911,"source:body-systems-britannica-cardiovascular":912,"source:body-systems-britannica-digestive":913,"source:body-systems-britannica-nervous":914,"source:body-systems-britannica-respiratory":915,"source:body-systems-wiki-homeostasis":916},"40eda343e2312ed1d0eed875e27ef482c65d948452c1be27522b9a4268085e71","3d6b0fe1b15255975a32b0fcd94e8019bc959ad45cbf12e136e86149549c6878","466896cc37735f48db03875fe9c9ce42fc8bcb7e5f937c9779d70513703b91bd","b164f45a2c8ca08f26c450768ff0231e113e9fe45381eddb34dc6d0548596c38","2a8ee4ac87460b4e1175a4bb13c96b03d577db06dde95670eb7fcfe4ad787899","8b7278004bfe64d7ad3e369fbfa62cada6c2ca19bdda240520d8cb96569d40a7","8b28eef54dd7327ab08588678c32674f0a6615182ae48d49d272ad156d0ad122","07d4488afad056b326893d48a8ecd74894b1e87c5b23c8f0bbd1268b77fdf252","dd8e7c0348ffd73f06866939a2804a19b80938f30a21e7c2b3e4ac922cb96f65","0e7d85baebb026691fbe6a4ef3c8aeaf14bd5c792553e4d59439acce7e4f81b3",{"state":918,"reviewer":919,"selfReview":101,"reviewedAt":920,"method":921},"approved","The library owner","2026-09-20T10:18:37.581Z","owner_bulk","preview-7e1cbbcc4f",1789899598687]