Predict it, then test it

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.

Real 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.

Every 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.

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.

Inside 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.

Notice 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.

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?

You 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.

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.

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.

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.

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.

Many 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.

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.

This 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.

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.

Two 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.

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.

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.

This 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.

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.