Every piece of the world about us can be a solid, a liquid or a gas. If it is a solid, the tiny particles that make up the material are locked together. If it is a liquid, then the tiny particles are all attached, but can move freely. But when it is a gas, the particles rarely touch at all and can move in any direction they want.

Because gases do not have their particles touching all the time, there are many fewer of them in a litre of space than there would be in a liquid. As a result, gases are much lighter than liquids. That is why gases rise out of liquids when they boil.

If you heat a gas, the particles move apart even more, and so they are even lighter. This is why a warm gas can rise through a cold one, and why hot air in a hot air balloon can rise up through the cold air around it.

If you heat a solid, it will often turn into a liquid. If you heat that liquid, it will boil, and turn into a gas. It works the other way round, too. Make a gas, for example, water vapour, colder and it will change into water droplets. Make it colder still and the water droplets turn into solid ice.

We make use of this fact when we want to transport gases as fuels. The giant gas-carrying supertankers that bring us our gas for cooking and heating, have containers which are very cold. As a result, the gas turns into a liquid and occupies much less space, and so it is cheaper to transport. When the liquid fuel leaves the container ship it can be allowed to warm, and then it will turn back into a gas and will flow along pipes to our homes. The gas in camping stove cylinders is also liquid, in this case made liquid by putting the gas under pressure.