  The fact that the nucleus is lighter than its constituents, seems rather
strange. How can the whole have a smaller mass than its components? To get some
intuition about how this is possible, one can look at a simpler more familiar
system exhibiting the same phenomena; the hydrogen atom. It turns out that the
same is true here; the hydrogen atom is a little bit lighter than the sum of
its constituents. This difference corresponds to the ionisation energy of
hydrogen. This observation allows a simple explanation for how this is
possible; the destructive interference between the electric fields of the
proton and electron causes a reduction in the energy of the electric field and
hence a reduction in the contribution to the mass of the system from the
electric field energy. The same explanation can be extended to the nucleus,
thus giving some intuition about how this mass reduction happens. In the
process, the idea of classical particles being ball-like objects with localised
properties, is challenged.
