If you can't get a copy of Why Does E=MC2 by Brian Cox and Jeff Forshaw try any textbook on special relativity. I think the Cox and Forshaw explanation is as simple as it can get. It all boils down to the fact that the speed of light in a vacuum is as fast as information can travel. To plagierize blatently:

A fencing instructor named Fisk

In duels was terribly brisk

So fast was his action

The Fitzgerald contraction

Foreshortened his foil to a disk

Oy!