In physics, thermal expansion is the likelihood of matter to change in volume in response to a change in temperature. When a substance is heated, its basic particles move around more quickly and by doing so generally maintain a greater average separation. Materials that contract with an increase in temperature are very uncommon; this effect is limited in size, and only occurs within limited temperature ranges. The degree of expansion divided by the change in temperature is called the material's coefficient of thermal expansion and generally varies with temperature.