Each heat difference within a body, or between different bodies, always leads to the transfer of heat, so that a temperature balance is obtained. This heat transfer can take place in three different ways: through conductivity, convection or radiation. In reality heat transfer takes place in parallel, in all three ways. Conductivity takes place between solid bodies or between thin layers of a liquid or gas. Molecules in movement emit their kinetic energy to the adjacent molecules. Convection can take place as free convection, with the natural movement that occurs in a medium or as forced convection with movement caused by, for example, a fan or a pump. Forced convection gives significantly more intense heat transfer. All bodies with a temperature above 0°K emit heat radiation. When heat rays hit a body, some of the energy is absorbed and transforms to heat. Those rays that are not absorbed pass through the body or are reflected. Only an absolute black body can theoretically absorb all radiated energy. In practice heat transfer is the sum of the heat transfer that takes place through conductivity, convection and radiation.