How about this: "If gold required no energy to mine it, it would be worth nothing. If you could simply have it upon a whim, even if its supply were limited, it still would have no value. Water and air are the most useful and necessary components of life and the economy, yet it requires little to no energy to acquire it, therefore it holds no value. This is why it's necessary to expend energy to create a quantity that we use to hold value."