E=mc² (Einstein's equation)

Einstein's equation, E=mc², shows that energy (E) and mass (m) are interchangeable; they are different forms of the same thing. The 'c' is the speed of light, a very large number, so even a tiny amount of mass can be converted into a huge amount of energy. This principle explains how nuclear reactions release vast energy from small amounts of matter, like in the sun or nuclear power plants. Essentially, mass can be thought of as stored energy, and under the right conditions, it can be transformed into real, usable energy.