Where did the mass ``go'' in the reaction we just discussed?
The answer is that the binding energy of the 
U
nucleus is substantially less negative than that of
the final products.
Remember that the gravitational potential energy
between two massive bodies is zero when they are
infinitely far apart and becomes more and more negative
as they get closer together? [Lower gravitational potential
energy for an object at a lower height?] Well, the strong
nuclear force that binds nuclei together has at least this much
in common with gravity: it is attractive (at least at
intermediate range) and therefore produces a potential
energy ``well'' into which the constituents ``fall'' when
we make up a nucleus.
The other thing to realize is that potential energy counts in the evaluation of the total relativistic energy of an object; and if the object is at rest, then its potential energy counts in the evaluation of its rest mass. As a result, we might expect the rest mass of a space ship to be slightly larger after it leaves the Earth than it was on Earth, simply because it has left the ``gravity well'' of the Earth. This is the case! However, the mass change is imperceptibly small in this case.