E=Mc2 Explained

"One of Einstein's great insights was to realize that matter and energy are really different forms of the same thing. Matter can be turned into energy, and energy into matter.
For example, consider a simple hydrogen atom, basically composed of a single proton. This subatomic particle has a mass of
0.000 000 000 000 000 000 000 000 001 672 kg
This is a tiny mass indeed. But in everyday quantities of matter there are a lot of atoms! For instance, in one kilogram of pure water, the mass of hydrogen atoms amounts to just slightly more than 111 grams, or 0.111 kg. "

http://www.worsleyschool.net/science/files/emc2/emc2.html