All materials are made up of atoms. At any temperature above(-273 degrees celsius the atoms will be moving. In a solid they will be vibrating in fixed positions, in a liquid they will be jostling past each other and in a gas they will be whizzing past each other at very high speeds. When a material is heated, the kinetic energy of that material increases and it's atoms and molecules move about more. This means that each atom will take up more space due to it's movement so the material will expand. When it is cold the kinetic energy decreases, so the atoms take up less space and the material contracts.