In the solid state, substances occupy the minimum amount of space possible with the notable exception of ice. The units which make up a substance can be called particles. The particles in a solid substance are densely packed, ie are very close to each other, and are not free to move around, hence a solid has its own particular shape. When a solid is turned into a liquid, the particles are able to move around but keep in close contact with each other, so a liquid takes the shape of the container it is held in. By moving they take up more space than when they were held rigidly in a solid, hence the volume occupied by the substance increases. On turning from a liquid into a gas the particles are free to move wherever they can, regardless of where the other particles are. Gases, therefore, occupy the whole of the space within which they are contained.