The total http://physics.about.com/od/glossary/g/energy.htm in a closed or isolated system is constant, no matter what happens. Another law stated that the mass in an isolated system is constant. When Einstein discovered the relationshipE=mc2 (in other words that mass was a manifestation of energy) the law was said to refer to the conservation of mass-energy. The total of both mass and energy is retained, although some may change forms. The ultimate example of this is a nuclear explosion, where mass transforms into energy.