the first law of thermodynamics is that energy can neither be created nor destroyed; it can change only from one form to another. the law forms the basis of the principle of conservation of energy. this means that anything that uses energy is changing the energy from one kind of energy to another. for example, exercising changes energy from food into kinetic (motion) energy. another example: in the sun (or any star), nuclear fusion changes mass into heat and light (electromagnetic radiation), which travels to earth and is used by plants to create food (chemical energy) via photosynthesis, which can be eaten by animals allowing them to move (kinetic energy). energy only ever changes its form; it is neither created nor destroyed. this is why perpetual motion machines do not exist and could never exist; it would break a fundamental law of physics.