According to the second law of thermodynamics, energy cannot be created or destroyed, just changed from one form to another. For example, frictional energy is turned into heat, and potential energy is turned into kinetic energy, but the total energy must be conserved. In many food systems, we can deal with a simplification of a complete energy balance and simply account for enthalpy changes of a material as it moves through a unit process. In this case, energy inputs (e.g., mechanical energy, kinetic energy, and potential energy) are not important, and we can simply compare the internal energy of a material within a system. These other energy terms will be discussed further in the fluid flow section, where we perform a mechanical energy balance and assume that changes in internal energy are not important.