The law of conservation of energy states that energy cannot be lost or gained but just converted from one form to another. For example, when a pendulum is elevated to a certain height it acquires a potential energy. When the pendulum is released that potential energy is converted to kinetic energy in the downswing and back into potential energy in the upswing. However, unless a source of energy is introduced into the system such as, for example, giving the pendulum a push, the pendulum will not reach a higher elevation than the one from which it was originally released. In the video below, uploaded into You Tube by Johnny Rico, Professor Walter Lewin from the Massachusetts Institute of Technology (MIT) puts his life on the line to illustrate this principle.