The term perpetual motion, taken literally, refers to movement that goes on forever. However, the term more generally refers to any close system that produces more energy than it consumes. Such a device or system would be in violation of the law of conservation of energy, which states that energy can never be created or destroyed, and is therefore impossible. The most conventional type of perpetual motion machine is a mechanical system which (supposedly) sustains motion while inevitably losing energy to friction and air resistance.