One corollary of E=mc^2 is that the rest mass of a bound state of fundamental particles will not equal the sum of the rest masses of the individual constituents. For example, the rest mass of a Helium-4 nucleus turns out to be less than the sum of the rest masses of the four nucleons of which it is composed. The difference is only about 0.7% but is famously quite a lot of energy when interpreted using E=mc^2.
It's a lot more fun if we go one level down though: protons are far HEAVIER than the sum of their constituents. Apparently almost 99% of the rest mass of a proton is the binding energy of the zero-rest-mass gluons that hold their three quarks together.
I think it's fun that the vast majority of the mass of ordinary matter, of which we are all constituted, is best explained as gluon binding energy via E=mc^2.
It's a lot more fun if we go one level down though: protons are far HEAVIER than the sum of their constituents. Apparently almost 99% of the rest mass of a proton is the binding energy of the zero-rest-mass gluons that hold their three quarks together.
I think it's fun that the vast majority of the mass of ordinary matter, of which we are all constituted, is best explained as gluon binding energy via E=mc^2.