In physics, heat is energy that spontaneously passes between a system and its surroundings in some way other than through work or the transfer of matter. When a suitable physical pathway exists, heat flows spontaneously from a hotter to a colder body. The transfer can be by contact between the source and the destination body, as in conduction; or by radiation between remote bodies; or by conduction and radiation through a thick solid wall; or by way of an intermediate fluid body, as in convective circulation; or by a combination of these. Because heat refers to a quantity of energy transferred between two bodies, it is not a state function of either of the bodies, in contrast to temperature and internal energy. Instead, according to the first law of thermodynamics heat exchanged during some process contributes to the change in the internal energy, and the amount of heat can be quantified by the equivalent amount of work that would bring about the same change. While heat flows spontaneously from hot to cold, it is possible to construct a heat pump or refrigeration system that does work to increase the difference in temperature between two systems. Conversely, a heat engine reduces an existing temperature difference to do work on another system.