Thermal energy is the sum of all the random kinetic energies of the molecules in a substance, that is, the energy in their motions. The higher the temperature, the greater the thermal energy. On the Kelvin temperature scale, thermal energy is directly proportional to temperature.

All matter is composed of molecules or, in some cases, just atoms. In gases and liquids, molecules are relatively free to move around. In a solid they are not so free to move around, but they can vibrate. Although molecules are microscopic, they do have some mass. Combining mass and speed gives them kinetic energy. Depending on the substance, the particles may interact with each other or their surroundings, in which case they would have potential energy along with kinetic energy. Summing the kinetic energy and potential energy of all the molecules gives the total energy of the substance. This energy...