In 1905, Einstein published two articles on the Special Theory of Relativity. He completed his first paper in June, on the properties of light and time. Then just three months later he finished a second, shorter article—essentially an addendum to his previous paper—describing a "very interesting conclusion" about energy. Einstein went on to present his findings mathematically: energy (E) equals mass (m) times the speed of light (c) squared (2), or E=mc2.

Source: American Museum of Natural History