In physics, heat is that amount of energy flowing from one body to another spontaneously due to their temperature difference, or by any means other than through work or the transfer of matter. 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 (like work) represents a quantity of energy being transferred between two bodies by certain processes, neither body "has" a definite amount of heat (much like a body in itself doesn't "have" work); in contrast, a body indeed has properties (state functions) such as temperature and internal energy. Thus, energy exchanged as heat during a given process changes the (internal) energy of each body by equal and opposite amounts. The sign of the quantity of heat can indicate the direction of the transfer, for example from system A to system B; negation indicates energy flowing in the opposite direction. 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.