Energy is the ability to bring about change or to do work.
Thermodynamics is the study of energy. The total amount of energy and matter in the Universe remains constant, merely changing from one form to another. In essence, energy can be converted from one form into another. This is also commonly referred to as entropy . A watchspring-driven watch will run until the potential energy in the spring is converted, and not again until energy is reapplied to the spring to rewind it.