Nearly every material, whether it is solid, liquid, or gas, expands when its temperature goes up and contracts when its temperature goes down. This property, called thermal expansion, makes a hot air balloon float, and the phenomenon has been harnessed to create thermostats that automatically turn a home furnace on and off. Railroads, bridges, and buildings are designed with this property in mind, and they are given room to expand without buckling or breaking on a hot day.
Thermal expansion occurs because a material's atoms vibrate more as its temperature increases. The more its atoms vibrate, the more they push away from their neighboring atoms. As the space between the atoms increases, the density of the material decreases and its overall size increases.
There are a few exceptions, but by and large, materials conform strictly to this principle. There is, however, a class of metal alloys called Invars (think invariable), that stubbornly refuse to change in size and density over a large range of temperatures.
