About.com explains that the law of conservation of energy states that energy cannot be created or destroyed. Energy can only be changed from one form to another, and the total energy of a system always remains constant.
The capacity of a physical system to perform work is known as energy. There are many different types of energy, including light, heat, electrical energy and mechanical energy. About.com states that when two objects collide and come to a rest, the resulting sound and heat is an example of the energy changing forms.
When Einstein discovered the equation of mass-energy equivalence, E = mc2, he realized that mass and energy are directly related, and one can manifest the other. Like energy, mass can change forms, but the total sum of it is constant. A strong example of this conversion of mass into energy is a nuclear explosion, according to About.com.
Because energy cannot be created, a perpetual motion machine of the first kind is considered to be an impossibility, because it is impossible to deliver an endless amount of energy to a system without an external energy supply. According to Wikipedia, it is unknown if the entire universe follows the law of conservation of energy.