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Thermal expansion is the tendency of matter to increase in length, area, or volume, changing its size and density, in response to an increase in temperature.
Substances usually contract with decreasing temperature, with rare exceptions within limited temperature ranges. Temperature is a monotonic function of the average molecular kinetic energy of a substance. As energy in particles increases, they start moving faster and faster, weakening the intermolecular forces between them and therefore expanding the substance.
When a substance is heated, molecules begin to vibrate and move more, usually creating more distance between themselves. The relative expansion divided by the change in temperature is called the material's coefficient of linear thermal expansion and generally varies with temperature.