In the field of statistical thermodynamics today, heat refers to the transfer of the thermal agitation of the particles making up matter. It is what happens, for example, when two materials of different temperatures come into contact: the hotter material releases energy to the cooler material in the form of heat, a process known as heat (thermal) transfer. Care should be taken to avoid confusing heat, or thermal energy transfer, with temperature. Temperature is a state function. It reflects on a macroscopic scale the microscopic agitation, or kinetic energy, of the particles of a material or system.