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.