Possible Duplicate:
How to explain $E=mc^2$ mass defect in fission/fusion
In order for energy to be released, mass has to be "lost", because mass is a form of energy.
In my science class, we are discussing fission and fusion of atoms, and since energy can be retrieved from these processes, I'm wondering where the mass loss is coming from.
Can somebody explain where mass is lost in these two processes?
