Substances absorb heat energy through three processes of heat transfer: convection, conduction and radiation. Convection transfers heat energy through fluid motion, such as in a liquid or gas. Conduction transfers interior heat through the tremors of atoms and molecules, and radiation transfers heat energy through electromagnetic waves.
Most instances of heat transfer involve all three forms of absorption, but one factor that remains constant is that heat always directs itself from a source of higher temperature to one of lower temperature. The propensity for matter to change in volume during heat exposure is known as thermal expansion. This is due to the heightened activity of the atoms and molecules, which reach greater points of separation as temperature increases.