Both Hubbertians and McKelveyans agree that an oil reserve is a physical stock: a finite pool of hydrocarbon molecules. To Hubbertians, the implication is clear: pump out too much and you will eventually empty it. How long you can pump depends primarily on the size of the pool. To McKelveyans, though, what matters most is not the size of the pool, but the capacity of the pump. The reason for this apparently counterintuitive belief is that a petroleum reserve is not, in fact, a subterranean pool, like the underground lake where Voldemort conceals part of his soul in the Harry Potter books, but rather an imprecisely defined zone of permeable, sponge-like rock that has petroleum in its pores.