 In their influential article, Bondi and Spurgin then examine a variety of cases of purported conversions of mass and energy. In each case, they show that the purported conversion of mass and energy is best understood merely as a transformation of energy. In general, Bondi and Spurgin argue, whenever we encounter a purported conversion of mass and energy, we can always explain what is taking place by looking at the constituents of the physical system in the reaction and examining how energy is proportioned among the constituents before and after the reaction takes place.