Just to elaborate a bit, for every bit of matter that something is comprised of, you get one hell of a lot of energy back from it, since the speed of light multiplied by itself is a pretty big number.
A person contains roughly 7,000,000,000,000,000,000 joules of energy.
Well I don't think Einstien just radomly though that squaring the speed of light was a good idea. It's been universally accepted that the equation correctly represents the total realisable energy from a chunk of matter.