All materials, solids, liquids or gases, are expanding if their temperatures are raised. In case of solids, the more increased the temperature is, the higher atoms vibrations become, they moving apart from each other and producing the expansion of solid. Hence, as the temperatures rise and the solid increases it's volume, the potential energy is also increasing. It is known that the changes of the volume of solid with temperature are less notable than the changes of volumes of a liquid.
Thermal expansion of solids can become an important problem, when dealing with long structures such as bridges, hence, the constructive solutions, such as expansion joints, become a must. Expansion joints are also the best solution for railroad tracks, which are built from steel that badly behaves under conditions of high temperatures.