Life & body

A brain rehearsing offline

Why Do We Dream?

In one stage of sleep the brain replays and files the day while the body is locked still and the fact-checking parts are turned right down.

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Roughly one bacterium per human cell

About Half of You Is Not You

Your body is roughly half bacteria by cell count, mostly living in your gut, and the famous ten-to-one version of that figure turned out to be wrong.

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Drowning out one signal with another

Why Does Scratching Feel So Good?

Your liver can hurt but it cannot itch, because there are no itch fibres below the surface. And scratching releases chemicals that bring it back.

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A planned demolition

Why Do Leaves Change Colour?

A tree times it by the length of the night, so a warm autumn still turns on schedule. The leaf never falls, either: a layer of cells forms and lets go.

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Light in, picture built

How Do Your Eyes See?

Eyes turn light into nerve signals. The picture you see is assembled afterwards by the brain — patched, corrected, and rather less direct than it feels.

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Blasting the nose clear

Why Do We Sneeze?

A sneeze is a reflex, not a decision: irritated nerves in your nose trigger a blast of air fast enough to clear them — and to spread germs remarkably well.

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A very fast crowd

Why Does a Pond Turn Green?

Sunlight is free, so stray fertiliser is what sets the limit — remove it and a handful becomes millions in a week, shading the pond floor dead.

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A defence aimed at nothing

Why Does Pollen Make Some People Sneeze?

An allergy is your immune system mounting a real defence against something harmless, and the symptoms are the defence, not the pollen.

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They only kill one kind of thing

Why Don't Antibiotics Work on a Cold?

Antibiotics attack structures only bacteria have, so they do nothing to viruses — and every unnecessary course breeds resistance.

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Pre-built, then inflated

How Does Bamboo Grow So Fast?

Bamboo grows a metre a day because the whole stem is built underground in advance, then simply pumped full of water and stretched out.

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Dead armour over a living pipe

What Is Bark For?

The rough outside is dead armour. The thin living layer just beneath carries the tree's food downwards, which is why cutting a ring round a trunk kills it.

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A windscreen wiper you never think about

Why Do You Blink?

Fifteen blinks a minute costs you half an hour of daylight a day, and screens halve that rate, which is exactly why they leave eyes feeling gritty.

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Four things in a river

What Is Blood Made Of?

Blood is mostly salty water carrying three cargoes: red cells that ferry oxygen, white cells that stand guard, and fragments that plug the leaks.

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The one blush you cannot fake

Why Do You Go Red When You Are Embarrassed?

Adrenaline opens the unusually wide blood vessels in your face — an honest signal precisely because you cannot start, stop or fake it.

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And a short list of exceptions

You Are Not Made of What You Were Made Of

Tissues replace themselves at wildly different speeds — days for gut lining, a decade for skeleton — and a few things are made once and never renewed.

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Stiff and springy together

What Are Bones Made Of?

Bone is a hard mineral laced with a stretchy protein, so it is strong without being brittle. It is also hollow, alive, and rebuilt thicker where loaded.

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Predicting, not just reacting

What Does Your Brain Do?

Your brain runs a constant prediction of what is happening and uses the senses to correct it. That is exactly why an illusion can fool you so completely.

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Fetching oxygen, dumping waste

Why Do We Breathe?

Breathing collects the oxygen that releases energy from food and dumps the carbon dioxide left over. The urge to breathe comes from the waste, not the air.

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Watching blood being cleared up

Why Does a Bruise Change Colour?

A bruise is blood spilled under unbroken skin, and its parade of colours is that blood being taken apart one stage at a time.

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Air that took a wrong turn

Where Does a Burp Come From?

Almost every burp is air you swallowed without noticing. Gas floats to the top of your stomach, and the way in is the only way out.

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The leaves are the spines

Why Does a Cactus Have Spines Instead of Leaves?

A cactus did not lose its leaves — it shrank them into spines to stop water escaping, and handed the job of feeding it to its green stem.

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The colour it refuses

Why Are Leaves Green?

Green is the one colour chlorophyll cannot use, so it is thrown back at you. The yellows sit underneath all summer, until autumn strips the green away.

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A heap that cooks itself

Why Does a Compost Heap Get Hot?

A heap has to reach about a cubic metre before its middle can hold heat, and once it does the warmth kills weed seeds and many plant diseases.

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A sudden gale, from inside

Why Do You Cough?

A cough is a pressure explosion: seal the airway, squeeze hard, then let go all at once, fast enough to scrape the whole tube clear.

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Being eaten by the invisible

Why Do Dead Things Rot?

Rotting is countless microbes digesting what has died from the outside in, unlocking nutrients that would otherwise stay stuck in the body forever.

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Something living in you, or something going wrong

Why Do You Get Ill?

Some illnesses are living things multiplying inside you and some are the body's own workings going wrong, and they need opposite treatments.

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Fluid that keeps going

Why Does Spinning Make You Dizzy?

Three loops of liquid in each ear sense turning, and when you stop the liquid does not, so your ears insist on a spin your eyes deny.

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Leaves cheap enough to keep

Why Don't Pine Trees Lose Their Leaves?

A waxy narrow needle survives a winter of frozen ground where a broad leaf cannot, so the pine is already working when the light returns.

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Demand outrunning supply

Why Does Exercise Make You Puff?

Working muscles burn oxygen far faster than resting ones, so breathing and heart rate climb to keep up. Puffing is the delivery system catching up.

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Normal, but not this fast

Is Extinction Normal?

Nearly every kind of animal that has ever lived is already extinct, so the alarming measurement today is the rate of it, not the fact of it.

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Older than flowers

Why Don't Ferns Have Flowers?

Ferns are far older than flowers and never changed: they scatter dust-fine spores, which need a film of water — hence a life in damp shade.

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A backstop for your fingertip

What Are Fingernails For?

Fingernails are a hard backstop that lets your soft fingertip press against something, making fine touch and picking things up possible.

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Four rings, one job

What Is a Flower Made Of?

A flower is built in rings, and the middle ring is the one that matters: once pollen arrives, that ovary swells into fruit and its eggs become seeds.

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Energy passed along

Who Eats Whom?

Every step of a food chain spends about 90% of its energy simply living, so a tenth passes upward. After four or five links there is nothing left.

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The food is the small part

What Gets Wasted When Food Is Thrown Away?

Throwing away a sandwich wastes the field, the water, the fuel and the year behind it — and about a third of all food grown is never eaten.

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Living things too small to see

What Are Germs?

Thousands of bacteria fit across a full stop, and one doubling every twenty minutes becomes millions in a day. Feeling ill is largely your own response.

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Moving the focus point

How Do Glasses Help You See?

Sharp vision means light landing exactly on the retina, and a millimetre of extra eyeball is enough to blur it. A lens moves the focus back again.

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A reflex for fur you lost

Why Do You Get Goosebumps?

A tiny muscle tilts each hair upright. On a furry animal that is real insulation; on us it is a leftover reflex with nothing much left to lift.

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Growing from the wrong end

Why Doesn't Cutting Grass Kill It?

Almost every plant grows from its tips, which is why cutting hurts them. Grass grows from the base of each leaf, so mowing removes only the oldest part.

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Cartilage turning into bone

How Do You Grow Taller?

Bones lengthen at soft cartilage zones near their ends, mostly during deep sleep, and when those zones harden for good your height is settled for life.

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A name that is probably wrong

Do Growing Pains Really Come From Growing?

Bones grow a few micrometres a day, which nobody could possibly feel — so growing pains are real aches with a badly misleading name.

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Dead rope, pushed out of a living hole

Why Does Cutting Hair Not Hurt?

A round follicle pushes out a straight strand and an oval one a curly strand, and grey arrives when the pigment cells at the root simply stop.

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Vibration turned into signals

How Do You Hear?

Sound shakes your eardrum, three tiny bones amplify the motion, and a coiled tube sorts it by pitch before hair cells turn it all into nerve signals.

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A pump squeezing

What Is a Heartbeat?

A heartbeat is a muscle squeezing blood around a one-way loop, about once a second for a whole lifetime. The sound is its valves snapping shut.

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Two wards, two death rates

How We Found Out Germs Exist

One ward killed five times as many mothers as the other, and handwashing fixed it twenty years before anyone could name what was washed off.

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Harvey, 1628

How We Found Out Blood Goes Round

William Harvey proved blood circulates by multiplying: the heart pumps far more blood in an hour than the body could possibly make.

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An empty tube, squeezing anyway

Why Does Your Stomach Rumble?

Your stomach does not stop working when it is empty. It sweeps itself out, loudly, and hunger itself is a hormone that peaks on a schedule.

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Guess everything, keep what works

How Does Your Body Fight a New Germ?

Most of those shapes will never match anything at all. The ones that do are kept afterwards, so meeting the same germ in twenty years takes hours.

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A recipe, copied twice

Why Do You Look Like Your Parents?

One version can mask the other completely, so a trait can sit silent in a family for generations and then turn up in a child who never met it.

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Seven metres of grabbing

Why Are Your Intestines So Long?

Ridges multiply the area by three, villi by thirty and each cell's own fuzz by twenty again. The squeezing works upside down, so astronauts can eat.

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Throw it all out, take back what you need

What Do Kidneys Actually Do?

Kidneys do not pick the rubbish out of your blood. They dump nearly everything small, then deliberately reclaim the water, salt and sugar.

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The chemistry room

What Is a Liver For?

Take two-thirds of a liver away and the rest grows back within a couple of months. No other large organ in you will do anything of the kind.

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Half a tennis court, folded up

Why Are Lungs Spongy Instead of Hollow?

A hollow bag would touch air only at its walls, so lungs split into millions of tiny sacs — around 70 square metres of surface, folded up.

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It does not, and that is the problem

How Does a Pill Know Where to Go?

A swallowed pill goes everywhere in your body and works only where its shape fits something, which is also exactly why side effects happen.

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No plumbing, no height

Why Is Moss Always Small?

Moss has no pipes to lift water, so every drop soaks from cell to cell — and soaking only works across millimetres, which sets its height.

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Only ever pulling

How Do Muscles Move You?

Muscles can only pull, never push, so they come in opposing pairs and use bones as levers to turn a short pull into a much larger movement.

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The tip of something bigger

What Is a Mushroom?

The mushroom is only the fruit. The real organism is a hidden network of threads that digests its food outside itself and quietly recycles the forest.

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An alarm, not a measurement

Why Does It Hurt?

Pain is not damage arriving at your brain but your brain deciding an alarm is worth sounding — which is why a paper cut can scream at you.

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Building food from air

How Do Plants Eat Light?

Plants build sugar out of air, water and light. A tree is made mostly of air — its carbon comes from carbon dioxide, not from the soil it stands in.

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It is holding your neck, not your head

Why Does a Pillow Help You Sleep?

Lying down leaves a gap between your head and the mattress. A pillow fills it, so no neck muscle has to hold your head up for seven hours.

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Pollen reaching the right place

What Is Pollination?

Pollination is pollen getting from one flower to another so that seeds can form, mostly carried by animals bribed with nectar to make the trip.

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Too much, in the wrong place

What Makes Something Pollution?

Nothing is pollution by itself. It becomes pollution when more of it arrives somewhere than that place can break down or carry away.

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Anchor and straw

What Do Roots Do?

Roots do three jobs at once: hold the plant down, drink water and minerals through microscopic hairs, and trade sugar with fungi for nutrients underground.

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A net, cast in seconds

How Does a Cut Stop Bleeding?

The scab is only a lid. Underneath it new skin creeps in from both edges, and when it meets in the middle the crust has nothing left to hold on to.

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Green, but not a plant

Is Seaweed a Plant?

Land plants all descend from one green ancestor. Seaweeds come from three separate lines, and many lift their fronds to the light on gas bladders.

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A plant and its lunch

What Is Inside a Seed?

A seed is a whole dormant plant packed with its own food store inside a tough coat, waiting for water to arrive and switch it back on.

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Muscles working for the heat, not the movement

Why Do You Shiver When You Are Cold?

Shivering makes your muscles do deliberately useless work, because a muscle is a poor engine and most of its effort comes out as heat anyway.

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More than a wrapper

What Does Your Skin Do?

Skin is four things at once: a barrier that keeps the world out, a thermostat, a sense organ, and a surface that entirely replaces itself as it wears away.

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Maintenance time

Why Do We Sleep?

Sleep is when the body repairs itself, the brain files what happened, and the whole thing gets flushed clean. Far too valuable to be optional.

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Catching actual molecules

How Does Smelling Work?

Around 400 receptor types, each waiting for a particular molecular shape, combine into trillions of smells — and into most of what you call taste.

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Taken apart, then absorbed

What Happens to Food You Eat?

Nothing is absorbed until it is molecule-sized. Acid unravels proteins, but the real work happens in seven metres of small intestine folded very small.

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Cooling by evaporation

Why Do We Sweat?

Sweat cools you only as it evaporates, because evaporation carries heat off your skin. Hence a breeze helps, and humid heat is so much harder to bear.

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Five basics, plus your nose

How Does Taste Work?

Your tongue detects only about five things. Everything else you call flavour is smell, arriving at your nose from the back of your own mouth.

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Three kinds of tear

Why Do We Cry?

Your eyes make three different tears: a constant film for maintenance, a flood to rinse irritants out, and a chemically different one for feelings.

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A toolkit in your mouth

Why Do Teeth Have Different Shapes?

Chewing works by multiplying surface area, and three tooth shapes split the job: chisels at the front, cones for gripping, millstones at the back.

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A saltiness detector

How Does Your Body Know You Need a Drink?

Thirst is not a dry mouth. A small patch of brain is measuring how salty your blood has become, and it switches off the moment water is on its way down.

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Several senses at once

How Does Touch Work?

Touch is not one sense but several — pressure, temperature, pain and stretch — and most of it reports change rather than steady contact.

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One line a year

Why Does a Tree Have Rings?

A tree adds one ring a year, with a dark line for the slow late growth. The widths record what the weather was doing long before anyone wrote it down.

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Some of it — and the sea makes more

Do Trees Make the Air We Breathe?

Trees release oxygen, but ocean plankton make about half the world's supply — and a forest that has stopped growing gives back roughly what it consumes.

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A rehearsal

How Do Vaccines Work?

A vaccine shows the immune system a harmless preview of a germ, so the real thing arrives to find a defence already rehearsed and waiting for it.

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Borrowing someone else's trunk

Why Do Vines Climb?

Building a trunk is enormously expensive, so a vine grows thin and fast, grabs a support somebody else built, and spends the rest on leaves.

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Two flaps, flapping very fast

How Do You Make Your Voice?

Whisper and the folds never touch — it is only rushing air, shaped the same way, which is why a whisper is still perfectly understandable.

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A judgement, not a species

What Makes a Plant a Weed?

Mint in a pot is a herb and mint in a border is a menace. Break a dandelion root and every piece regrows, which is why digging can spread it.

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A bag that reports on itself

Why Can't You Just Hold It Forever?

Two tubes trickle into it continuously, the first polite signal arrives at about a third full, and cold weather really does make you produce more.

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