 Following Geroch, we can begin to explain the physical significance of Einstein’s equation \(E_o = mc^2\) by considering a very simple physical system. Imagine, as Geroch suggests, a brick being heated or a battery being charged (Geroch 2005, p. 198). Suppose we consider these objects in the inertial reference frame in which they are at rest. As the brick is heated, or the battery charged, it absorbs an amount of energy \(E_o\) as measured in its rest frame. Einstein’s equation tells us that the mass of the brick, or battery, after it absorbs an amount of energy \(E_o\) is increased exactly by an amount \(E_o /c^2\). The value of the mass of the hot brick or charged battery is greater than it was before it absorbed energy. So, for example, it takes just a little bit more force to move the charged battery than it did to move the uncharged battery. How much is “a little bit more”? 