Heat can be transferred through conduction, convection, and radiation. Conduction involves the transfer of heat between objects in direct contact through collisions of atoms and molecules. It occurs at a rate directly proportional to the temperature gradient and the area, and inversely proportional to the material's thickness and thermal conductivity. Convection involves the transfer of heat by the circulation or movement of fluids such as gases and liquids, and can be natural or forced. Radiation is heat transfer between two bodies or a body and its environment without requiring physical contact. It occurs through electromagnetic waves.