Most substances exist in one of three states: solid, liquid or gas. They change from one state to another when they are heated or cooled. Water is a liquid at room temperature, but if you freeze it, it becomes a solid (ice). When boiled it turns into a gas (water vapour or steam).

Molecules

The molecules in a solid are tightly packed and do not move about much. This is why solids have a fixed shape. The molecules in a liquid are not all joined together. They move about more and spread out to fill the shape of their container. The molecules in a gas are far apart. They move about a lot and fill the container they are in. This is why gases have no fixed shape.

Crystals

In many solids, the molecules are arranged in regular patterns called crystals. Crystals are flat-faced, straight-sided shapes. They fit together to form much bigger, regular shapes. Snowflakes, salt, sugar, minerals in rocks and many metals are made up of crystals.