Every so often hear things similar to: The mass lost is due to the energy produced. Mass becomes energy from the equation E=mc2. We seem to have two competing ideas: 1) E=mc2 2) E + mc2 = constant. A) In equation 1) energy and mass are equivalent (up to a units conversion factor). B) In equation 2) the sum is conserved--one is changed into the other. Any disagreement?