Heat is the form of energy transferred between two materials that are at different temperatures. Heat always flows from the material with higher temperature to the material with a lower temperature until thermal equilibrium is reached. Since heat is a form of energy, the SI unit of heat is the joule. When a substance is heated, its molecular movement increases. Due to the average distance between the molecules also increases, thereby increasing the volume of the substance, i.e., expanding the substance. However, the movement of molecules depends upon the state of matter. The biggest example of heat energy in our solar system is the sun itself. The sun radiates heat to warm us up on the planet earth. When the burner of a stovetop is boiling, it is a source of heat energy. Automobile fuels such as gasoline are sources of heat energy, as is the hot engine of a race car or a school bus. As a form of energy, heat has the unit joule (J) in the International System of Units (SI). The standard unit for the rate of heat transferred is the watt (W), defined as one joule per second.