Principle of Conservation of energy:

Energy can neither be created nor destroyed but it can transformed from one form to another.

Think about the wound-up spring that is accelerating a clockwork toy. Strain energy changes into kinetic energy and heat (the heat energy passes into the toy and makes it slightly warmer). The total amount of kinetic and heat energy produces equals the amount of strain energy that disappears.

Energy can be measured , and the unit to measure all forms of energy is the JOULE. If the spring loses 20 joules of strain energy , the 20 joules of energy must appear in other forms-Kinetic and heat in this case. There can be no overall loss of energy. Energy cannot be destroyed.