The reactants in a reaction are the materials which are present before the reaction begins. The products are the materials which are present after the reaction has occurred.

Consider America's favorite equation:

Since E (energy) and M (mass) are on opposite sides of the equation, we know that they can be interchanged somehow, in a nuclear reaction. (The factor of c squared is a conversion factor, which takes care of turning units of energy into units of mass. This is like converting twelve eggs into a crate, or six tall women into a single volleyball team.)

What has occurred in a reaction such as the fusion process in the solar core, one in which the reactants weigh less than the products?