Heat & cold

99.8 per cent nothing

The Solid That Is Almost Entirely Air

Aerogel is a gel with the liquid removed and the solid skeleton left standing — a feat of drying rather than chemistry, and the best insulator we can make.

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The part that would not burn

What Is Left When Something Burns?

Barely one part in a hundred of a burnt log stays behind, and that pale grey pile is the soil minerals a tree spent its whole life collecting.

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Fire welds the grains together

Why Does Baking Mud Make Stone?

Firing clay drives out the water locked inside and heats the grains until their edges fuse. Soft mud comes out behaving, for all purposes, like stone.

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Where roast smells come from

Why Does Food Turn Brown When It Cooks?

Above about 140 degrees, sugars and proteins start reacting with each other and build hundreds of colour and flavour molecules that were not there before.

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A skin of glass

Why Doesn't a Glazed Pot Leak?

A fired pot is porous and leaks; glaze is powdered glass melted over it, and it has to melt low enough and shrink by exactly the right amount.

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Burning wood twice

Why Does Charcoal Burn Hotter Than Wood?

Charcoal is wood already half burned: baked without air until only carbon remains, so everything that would have made smoke has gone.

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Rot it, roast it, grind it

How Does a Bitter Bean Become Chocolate?

A raw cocoa bean tastes horrible. Chocolate exists because somebody let the beans ferment first, then dried, roasted and ground them.

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A narrow window of heat

Why Does Custard Suddenly Scramble?

Egg proteins unfold and tangle to thicken a custard over a few degrees, then clump and squeeze the liquid out over the next few. The margin is very thin.

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Mostly air, held in a shape

Why Is Foam So Light?

Packaging foam is 95 to 98 per cent air. Trapped air cannot circulate, so it insulates — and thousands of bubble walls buckling is what absorbs a knock.

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Oil goes far hotter than water

Why Does Frying Taste Different From Boiling?

Water stops dead at 100 degrees, so boiling can only soften. Oil sails on to 180, and that is where browning, crisping and most of the flavour happen.

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A mesh built from fruit

Why Does Jam Set?

Long pectin chains from the fruit link into an invisible mesh once acid and sugar allow it, and the sugary liquid is held still inside it.

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Melted, then cooled too fast to crystallise

How Do You Turn Sand Into a Window?

Glass is melted sand cooled so fast that its atoms freeze in a jumble, never getting the chance to line up into the crystals they would prefer.

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How hard the atoms hold on

Why Do You Need a Furnace to Melt Iron?

The tipping point is sharp: a degree below it the lump holds its shape. Tungsten atoms grip hardest of all, which is why old bulbs used tungsten wire.

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A sensor told a lie

Why Does Mint Feel Cold?

Mint cools nothing at all. Menthol holds your cold sensor open, so the nerve reports cold while the thermometer never moves. Chilli does the same trick.

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A chemical that lies to your nerves

Why Are Some Peppers Hot?

A chilli is not actually hot. It carries a chemical that opens the nerve reporting high temperature, so the burn is a signal about nothing.

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A fruit that eats you back

Why Does a Pineapple Sting Your Mouth?

The enzyme that tingles is the one that tenderises steak and stops a jelly setting — and every diamond on the skin was once its own flower.

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Starch drinking the water

Why Does Porridge Go Thick?

Oats and water go into the pan as two things and leave as one, because heated starch grains swell, burst and tangle into a water-holding mesh.

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The same temperature, treated gently

Why Do Cooks Simmer Instead of Boil?

A rolling boil is not one degree hotter than a gentle simmer. All the extra violence buys is faster evaporation and food knocked to pieces.

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The bits that did not get to burn

Why Does a Fire Make Smoke?

Smoke is fuel that never finished burning: tiny solid specks that failed to meet enough oxygen, carried off by the hot air rising above the fire.

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Cooking without soaking

Why Does Steaming Keep the Flavour In?

Boiled food loses its flavour and vitamins into the water. Steam cooks at the same temperature but gives them no water to dissolve into.

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A pinch of carbon changes everything

What Makes Steel Stronger Than Iron?

Add about one percent carbon to iron and it becomes several times stronger, because the carbon jams the layers that used to slide past.

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Cold, sweet and mostly water

Why Is Watermelon So Refreshing?

Watermelon is over nine-tenths water with sugar and salts in it, which is close to what a body actually needs after sweating.

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The wick drinks its own fuel

Why Does a Candle Burn for Hours?

The wick is not the fuel and solid wax will not catch: the flame melts the wax, the wick lifts it, and what actually burns is the vapour.

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Glowing gas, not glowing wood

What Actually Is a Flame?

A flame is not the wood burning. It is gas driven out of the wood by heat, glowing as it meets oxygen — which is why heat, air or fuel each put it out.

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It is mostly trapped air

Why Is Wool So Warm?

Wool traps a great deal of still air in its crimped fibres, and still air carries heat badly — though its one-way scales shrink in a hot wash.

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Closer to the sun, and colder

Why Is It Colder Up a Mountain?

Climbing takes you nearer the Sun and makes you colder, because air is warmed from the ground up and cools again as it rises and expands.

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The line the sunlight hits squarely

What Is the Equator?

The equator is the line halfway between the poles, where sunlight lands most directly and days stay the same length all year.

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Escaping one at a time

Where Does a Puddle Go?

No boiling required. The fastest-moving molecules at the surface simply escape into the air — which is also why the water left in the puddle gets cooler.

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Air letting go of water

Why Is the Grass Wet in the Morning?

Grass cools overnight until the air touching it can no longer hold its water, which is left behind as dew, or as frost if it is cold enough.

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A kettle with a narrow neck

Why Does the Ground Shoot Boiling Water?

Water works down through cracks to rock heated by magma two kilometres below, then waits there hotter than boiling for something to let the pressure go.

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A blanket that lets light in

How Does the Air Keep Earth Warm?

The light coming in and the heat going back out are different waves — some gases ignore sunlight and absorb infrared, worth 33 degrees.

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Air that will not leave

What Makes a Heatwave?

A heatwave is not stronger sunshine. It is a dome of sinking air blocking cloud and wind, so the same heat accumulates day after day instead of leaving.

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Sorted by weight

What Is Inside the Earth?

The young Earth was molten, so heavy iron sank and light rock floated. Earthquake waves passing through confirm the layers are still arranged that way.

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Hot, and solid anyway

Why Is the Inside of the Earth Not Liquid?

The Earth's inside is hot enough to melt but held solid by pressure — so magma is made by dropping the pressure or adding water, not by adding heat.

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Ice you never see

Why Does Rain Begin as Snow?

Most rain starts as ice, because an ice crystal grows by stealing water vapour from the droplets around it — far faster than droplets can grow by bumping.

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Warm and windy beats damp and still

Why Do Puddles Dry Slower Some Days?

The same puddle can vanish by lunchtime or sit there all day, because molecules land back into it almost as fast as they escape.

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Nothing is permanent

Can a Rock Turn Into a Different Rock?

Weather grinds rock to grains, burial presses the grains back into stone, and deeper heat cooks that stone into something new without ever melting it.

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Land and sea disagree

Why Is It Windy at the Beach?

The risen air spreads out high up, drifts back over the water and sinks again: land, sky, sea and round, a slow wheel turning all afternoon.

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The ground is still catching up

Why Is the Hottest Month Not the Longest Day?

Midsummer is peak sunshine, but ground and sea keep taking in more heat than they lose, so the hottest weeks arrive some weeks afterwards.

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A flake's journey down

Why Does It Sometimes Rain and Snow at Once?

Almost all rain begins as snow. What lands on you is decided in the last kilometre, by the warm and cold layers the flake happens to fall through.

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Pressure finding a way out

Why Do Volcanoes Erupt?

Molten rock is lighter than the rock above it, so it rises. Dissolved gas expanding on the way up is what turns that rise into an eruption.

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Fire as a gardener

Why Do Some Forests Need to Burn?

Some cones are glued shut with resin that only a fire can melt, so the seeds drop exactly when the ground has been cleared and made fertile.

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Air evening things out

What Makes Wind Blow?

Every temperature difference becomes a pressure difference, and wind is crowded air pushing towards the emptier place. Earth's spin makes it spiral.

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Losing your warm coat of air

Why Does Wind Make You Colder?

Your body heats a thin layer of air around your skin, and wind keeps stealing it. That is why you lose heat far faster than the thermometer suggests.

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Water turning to gas inside itself

What Are the Bubbles When Water Boils?

The bubbles are water itself, turned to vapour deep in the pan rather than air escaping. Once it boils the temperature stops rising, however high the heat.

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A wire crossed

Why Does Ice Cream Give You a Headache?

Press your tongue flat against the roof of your mouth and it stops. The nerve rarely hears from the palate, so it blames the forehead instead.

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Ice leaving without melting

What Is Freezer Burn?

Ice can leave your food without ever melting, turning straight to vapour and refreezing on the packet. Months of that leaves the food dry and stale.

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Locking into a pattern

What Happens When Water Freezes?

Freezing does not just harden water, it rearranges it into an open hexagonal lattice. That extra room is why ice expands, floats, and keeps lakes liveable.

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An honestly unfinished answer

Hot Water Sometimes Freezes First, and Nobody Is Sure Why

The Mpemba effect — hot water sometimes freezing before cold — keeps being reported, keeps being disputed, and after two thousand years is unexplained.

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Heat always moving in

Why Does Ice Cream Melt?

Heat always flows from warmer to colder and never the other way, so a warm room melts ice cream — and melting takes a surprising amount of energy.

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Lowering the freezing point

Why Does Salt Melt Ice?

Salt does not warm anything. It gets in the way of water molecules rejoining the ice, so the freezing point drops and ice below zero turns to liquid.

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It steals heat faster

Why Does Metal Feel Colder Than Wood?

Metal and wood in the same room are the same temperature. Metal only feels colder because it pulls heat out of your hand very much faster.

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Waves that overlap themselves

Why Does a Microwave Heat Unevenly?

Microwaves bounce off the metal box and overlap each other, making fixed hot and cold bands. The turntable and standing time are there to even them out.

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Steam bursting out

Why Does Popcorn Pop?

A kernel is a waterproof shell with water sealed inside. Heat turns that water to high-pressure steam until the shell can no longer hold it in.

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Stopping the heat walking out

Why Does a Lid Make Water Boil Faster?

Most of the heat leaving an open pan walks out as steam. A lid sends it back, so almost everything the hob supplies goes into the water instead.

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Making water boil hotter

How Does a Pressure Cooker Cook Faster?

Trapping the steam raises the pressure, and higher pressure pushes water's boiling point up to about 120 degrees. Hotter water simply cooks faster.

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Air losing its grip on water

Why Do Windows Fog Up?

Warm air carries water you cannot see. Cold glass chills the air touching it until it can no longer hold that water, and the surplus appears as drops.

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How tightly the molecules hold

Why Are Some Things Solid and Others Runny?

Solid, liquid and gas are the same molecules with the same attractions — what changes is whether heat energy or that attraction is currently winning.

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Metal and protein shaking hands

Why Does Food Stick to the Pan?

Food sticks because its proteins genuinely bond to hot metal. It lets go on its own once a dry crust has formed, or never bonds if oil keeps the two apart.

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Sugars and proteins reacting

Why Does Toast Go Brown?

Toast is not just drying out. Heat drives sugars and proteins to react into new brown, aromatic compounds — the same reaction as searing a steak.

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Cooling and catching

Why Do We Yawn?

Yawning seems to cool the brain rather than fetch oxygen, and it spreads between people along the circuits that carry empathy — strangers catch it least.

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Turning speed into heat

How Does Rubbing Stop a Car?

Brakes do not remove a car's energy, they convert it. Pads squeeze a spinning disc and all that movement leaves as heat the disc sheds into the air.

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A shove converted to a spin

How Does a Bang Turn a Wheel?

Burning fuel shoves a piston down a tube, and a crankshaft bends that straight shove into the round motion the wheels actually need.

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Carrying heat out, not cold in

How Does a Box Make Itself Cold?

There is no such substance as cold. A fridge squeezes a gas hot to dump heat out behind it, then lets it expand cold to soak up some more.

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Something has to break first

Why Do Fuses Blow?

A fuse is the weakest part of a circuit on purpose: a thin wire that melts before the cable behind it can ever get hot enough to burn.

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A fan and a hot wire, in that order

What Is Inside a Hairdryer?

A hairdryer is a fan and a coil of thin wire — and hair dries because moving air carries water away, not because the air is hot.

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The same bag, holding less

Why Does Hot Air Lift a Balloon?

Heating the air drives some of it out of the open mouth, so what is left inside is about a fifth lighter than the air outside the balloon.

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Steam that flips a switch

Why Does a Kettle Switch Itself Off?

Steam runs down a hidden tube to a strip of two bonded metals, which bends as it warms until it snaps the power off behind your back.

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A wire too hot to stay dark

How Does Electricity Become Light?

Only the filament is thin, so the current squeezes there and pours out white light near 2,500°C. An LED skips the heat entirely.

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Twisting water molecules

Why Does a Microwave Heat Food, Not the Plate?

A microwave flips a radio field billions of times a second and only water molecules follow, so wet food heats and a dry plate does not.

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Splitting atoms to boil a kettle

How Does Nuclear Power Work?

A nuclear station is an ordinary steam power station with a very strange fire: split atoms make the heat, control rods hold the rate steady.

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Dry heat, and a reaction

Why Does an Oven Brown Food?

Boiling water and an oven reach similar temperatures and only one makes toast, because dry heat lets the surface climb past a hundred degrees.

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Almost all of them boil water

How Does a Power Station Work?

Coal, gas and nuclear stations look nothing alike and all do one thing — boil water, so the steam spins a magnet inside coils of wire.

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A switch that feels the room

How Does a Heater Know When to Stop?

A thermostat is just a switch that temperature opens and closes, which is why a room drifts slowly above and below the number you set.

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The steadiest place around

Why Do Animals Dig Burrows?

A metre of soil holds one temperature through a heatwave and a frost alike, and the door is narrow — both worth an enormous amount of digging.

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A comb and a scraper

How Does a Cricket Chirp?

A cricket does not chirp with its mouth. It scrapes a hard wing edge over a comb of ridges, and listens with eardrums in its front legs.

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A bird that sits on water all day

Why Does a Duck Stay Dry?

A duck stays dry with zipped, oiled feathers over trapped air, and keeps its feet deliberately cold so that it loses no heat through them.

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A nose with forty thousand muscles

What Can an Elephant Do With Its Trunk?

An elephant's trunk is a boneless nose and upper lip with tens of thousands of muscles, used to breathe, drink, grab, smell and talk.

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Holding onto still air

Why Do Animals Have Fur?

Fur works by trapping a layer of still air against the skin, and animals fluff it, shed it and grow it back as the seasons change.

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Turning the engine down

Why Do Animals Hibernate?

Hibernation is not deep sleep but a controlled shutdown — body temperature and metabolism dropped far enough that stored fat can outlast a foodless winter.

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Borrowing warmth instead of making it

Why Do Lizards Sit in the Sun?

A lizard eats a small fraction of what a mammal its size needs, because it buys its warmth from the sun instead of paying for it with food.

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Four layers and a good idea

How Do Penguins Stay Warm in the Snow?

An emperor penguin stands through months of the coldest dark on Earth without eating, on trapped air, fat, cold feet and a rotating huddle.

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Cooling down without sweat

Why Do Pigs Roll in Mud?

A pig can barely sweat, so it wears mud instead. Mud dries slowly enough to carry heat away for hours, which plain water could never manage.

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Radiators that also listen

Why Do Rabbits Have Such Big Ears?

Related animals in hot places grow bigger ears, legs and noses the world over — a desert jackrabbit's run to about a quarter of its own body length.

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Wool that never stops

Why Do Sheep Need a Haircut?

A wild sheep sheds its coat every year. Thousands of years of breeding took that shedding out of farm sheep, so somebody now has to do it for them.

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The biggest animal there has ever been

Why Is a Whale Not a Fish?

A blue whale is longer than three buses and would drown if it could not surface: a warm, milk-feeding mammal that went back to the sea.

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You would not notice for eight minutes

What If the Sun Went Out Tonight?

The light takes eight minutes and twenty seconds to arrive, and so would the news — after which the freeze takes days rather than seconds.

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The strangest calendar

A Day on Mercury Lasts Longer Than Its Year

It laps the Sun in 88 days but takes 176 to get from one sunrise to the next, and radar has found ice in craters the sunlight never reaches.

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Not a star at all

What Is a Shooting Star?

A grain of dust hits our air at tens of kilometres a second and burns out 80 km up. The glow is not friction — air squeezed that hard gets white hot.

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Not friction

Why Do Spacecraft Get So Hot Coming Back?

A capsule coming home at eight kilometres a second is not rubbed hot by the air. It squashes the air in front of it, and squashed air glows.

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Both, and neither

Is Space Hot or Freezing?

Space is neither hot nor freezing in any useful sense. Temperature needs particles, and there are almost none — objects there heat and cool by light alone.

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From a cold cloud

How Does a Star Get Started?

A star begins as a cold cloud of gas falling in on itself, and switches on when the squeezed centre finally reaches about ten million degrees.

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Colour is temperature

Why Are Some Stars Red and Others Blue?

A star's colour is a thermometer you can read from a hundred light years away: red is the cool end, blue-white the hot end. It says nothing about size.

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Five billion years from now

What Will Happen to the Sun?

The Sun will not explode. It will swell into a red giant, swallow the inner planets, shed its outer layers and end as an Earth-sized ember.

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Not burning at all

What Is the Sun Burning?

The Sun is not on fire. Its own weight crushes hydrogen into helium, and the scrap of mass lost in that swap comes back out as light and heat.

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A blanket that will not lift

Why Is Venus Hotter Than Mercury?

Venus sits further from the Sun than Mercury yet runs far hotter, because its thick carbon dioxide blanket traps heat airless Mercury loses.

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Jostling passed along

Why Does a Metal Spoon Get Hot?

No atom ever travels along the spoon; each stays put and passes the jiggle on. Metal feels cold only because it pulls heat out of your hand fast.

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It never goes anywhere

Where Does Energy Go When Things Stop?

Energy is never used up when something stops. It has only changed form, and every change spills a little as heat that is hard to use again.

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Why things stop

What Is Friction?

Every surface is rough close up. Friction is those roughnesses catching and grinding on each other, quietly turning motion into heat as they go.

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Everything is shaking

What Is Heat?

Heat is not a substance you add. It is how fast the atoms are jiggling, which is why a thermometer is really a gauge of atomic speed.

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Water breaks the rules

Why Does Ice Float?

Almost everything shrinks as it freezes. Water does the opposite, locking into an open, gappy lattice that takes up more room — so the solid floats.

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It makes nothing — it slows

Does a Coat Make You Warm?

A coat makes no heat of its own. It traps still air, which slows the escape of the warmth your body is already producing every second.

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A thin film of water

Why Is Ice So Slippery?

Ice carries a film of liquid water well below freezing, because its top layer of molecules has nothing above to hold it in place. That film is the slip.

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About a hundred watts of it

You Are Glowing Right Now

Warm objects radiate light whose colour depends on their temperature — yours is infrared, invisible to us and perfectly obvious to the right camera.

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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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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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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 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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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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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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The line carries on

How Can a Number Be Less Than Nothing?

Negative numbers are just the number line carrying on past zero instead of stopping there. The minus sign says which side of zero you are standing on.

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A scale built around water

Why Does Water Freeze at Zero?

A scale needs two events anyone can reproduce, so Celsius took water freezing and boiling and split the gap into a hundred convenient steps.

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