Energy is the measured ability to make change. It is measured in joules (), which can be broken down into the SI units . It comes in two forms: potential energy, and kinetic energy. Examples of potential energy include elastic energy, chemical energy, and gravitational energy; examples of kinetic energy include thermal energy, electromagnetic (light) energy, electrical energy, and motion energy (often just called kinetic energy). Work, which is often confused with motion energy, is the change in motion energy from one instance in time to another.