Every material you encounter in your daily life is solid, liquid or gas. These are known as the three states of matter. There is a fourth state known as plasma, but this can only exist at extremely high temperatures, and is therefore exceedingly rare on Earth. What defines a substance as a solid, liquid or gas may seem obvious at first, but when you think about substances like toothpaste, margarine or jelly, their states are not as obvious. For something to be classed as a solid, it has to have a fixed volume and a fixed shape. Solids are not easily compressed and do not flow. Liquids also have a fixed volume but do not have a fixed shape. They take the shape of their container from the bottom upward if they are inside one, and they flow when they are poured out. Like solids, liquids are not easily compressed, which is why they are used in hydraulic brakes. Gases, on the other hand, are highly compressible. They have no fixed shape, no fixed volume, and they flow easily, taking up all the available space they can. Every nook and cranny in your house is filled with gas.