The property of a body that determines the direction of heat flow when the body is in contact with another body is the temperature. Heat naturally flows from a body at a higher temperature to a body at a lower temperature. Temperature is a measure of the average kinetic energy of the particles in a substance. When two objects at different temperatures come into contact, heat energy tends to transfer from the warmer object to the cooler one until they reach thermal equilibrium (they are at the same temperature). This is known as the second law of thermodynamics, which describes the direction of heat flow.