With the famous news of the LHC, I had did a little research about how it works. Apparently, it works via energy mass equivalence. After dancing around the net, I had a brief idea of how E = MC^2 came about. But still, it isn't very clear. Exactly how does the speed of light come into play? I know C meant constant, and speed of light just happens to be that constant, but how did we arrive on that constant? What makes it so sure that this C^2 is the relationship? Would appreciate a layman answer, preferably with less maths.