FT - If you heat something it gains thermal energy and all forms of energy have mass, according to the equation. Therefore the mass increases by an infinitesimal amount. This is not saying that the quantity of matter increases. E=mc^2 not only means the matter can be converted into energy (and vice versa) but also that they are equivalent; in other words both have mass and this means that it will take a slighly great force to throw a hot potato than a cold one.