Energy transformation or energy conversion is the process of changing one form of energy to one other. In physics, the term energy describes the capacity to produce certain changes within any system, without regard to limitations in transformation imposed. Changes in total energy of systems can only be accomplished by adding or removing energy from them, as energy is a quantity which is conserved (unchanging), as stated by the first law of thermodynamics. Mass-energy equivalence, which rose up from special relativity, states that changes in the energy of systems will also coincide with changes (often small in practice) in the system's mass, and the mass of a system is a measure of its energy content. The process of something happening (forming).