Look around the room. A wooden chair, the water in a glass, the air you are breathing right now — they seem like completely different things. But almost every bit of stuff in the whole world is doing one of just three things. It is being a solid, a liquid, or a gas.
These are called the three states of matter — the three ways that stuff can be. The chair is a solid, the water is a liquid, the air is a gas. And the secret to all three is the same: everything is built from tiny bits, far too small to see, and the whole difference is just how those bits are packed and how much they move.
A brick, a spoon, a coin, an ice cube, your bones — a solid holds on to its own shape and its own size wherever you put it. Carry a brick into the kitchen, the garden or the bath and it is still exactly the same brick-shaped brick. It does not flow, it does not spread out, and it will not squash down into a smaller brick however hard you press.
Here is why. Inside a solid the tiny bits are packed tightly together and locked into a fixed pattern, like people holding hands in a crowd so squashed that nobody can move away. They are not completely still — each bit jiggles a little on the spot, like someone shivering while staying in exactly the same place — but they cannot swap places or wander off. That locked-in pattern is what gives a solid its firm, unchanging shape.
Water, milk, juice, oil, runny honey — a liquid flows. Pour it into a tall thin glass and it makes a tall thin shape; pour that same juice into a wide flat bowl and it spreads into a wide flat shape. A liquid always takes the shape of whatever holds it.
But here is the clever part: even though the shape changes, the amount stays exactly the same. A cupful of water poured into a bowl is still a cupful — pour it back and it fills the cup again. A liquid also always sinks to the bottom and settles with a smooth flat top.
Inside a liquid the bits are still touching each other, packed almost as close as in a solid — but they are not locked in a pattern. They can slide and roll past one another, like people in a crowd shuffling past each other to move around. That sliding is what lets a liquid flow and pour and take any shape, while still keeping all its bits close together.
The air in a balloon, the steam over a boiling kettle, the helium in a party balloon — a gas does not settle at the bottom and it does not keep any shape at all. Instead a gas spreads out to fill every bit of space it can reach, right up to the top and into every corner. Let a little gas loose in a big room and it will quietly fill the whole room.
In a gas the bits are far apart with lots of empty space between them, and they zoom about freely in every direction, bouncing off each other and off the walls like a room full of tiny bouncy balls that never slow down. Because there is so much empty space between them, a gas is also the one state you can squash: press the bits closer together and they fit into a smaller space, which is exactly how a bicycle pump crams lots of air into a small tyre.
Here is a surprise: ice, water and steam are not three different things at all. They are the very same stuff — water — just being a solid, then a liquid, then a gas.
Take an ice cube from the freezer: it is water being a solid, hard and keeping its shape. Leave it on the table and it melts into a puddle: now the same water is a liquid, flowing and taking the shape of the table. Heat that water in a pan until it boils and it turns to steam: the same water again, now a gas, spreading up and away into the kitchen. Nothing was added and nothing was thrown away — the bits only unlocked, then spread out. Cool the steam back down and it becomes water again, and freeze that and you have an ice cube once more. Round and round, always the same water.
If someone hands you some stuff, you can work out which state it is with three simple questions:
Solids you meet all day: a table, a shoe, a marble, a biscuit. Liquids: water, tea, shampoo, soup. Gases: the air around you, the steam from a shower, the bubbles fizzing out of a lemonade.
Here is the same jar three times, holding the same number of bits. Switch between the states and watch where the bits go: a neat, tightly-packed block for the solid, a puddle touching at the bottom for the liquid, and bits spread far apart through the whole jar for the gas. Nothing is ever added or taken away — only the arrangement changes.
You can pour a liquid — but can you pour a gas? Mostly gases are too light and eager to stay where you put them: they just spread out and float off. That is why you can smell dinner from another room. Tiny bits of gas from the cooking pot go zooming off in every direction, spreading through the whole house until some reach your nose.
But one gas is a show-off. Carbon dioxide — the gas that makes fizzy drinks fizzy — is heavier than ordinary air. Fill a jug with it (invisibly) and you really can tip it out over a burning candle. The heavy gas pours down like an invisible waterfall, pushes the air away from the flame, and the candle goes out — even though you poured "nothing" you could see. A gas is real stuff, and heavy enough to pour.