This is what we first learned in calculus. As we get to smaller and smaller and smaller changes in quantity, we essentially take the limit as our change in quantity approaches zero. That's how we get the instantaneous change. So one way to think about it, one way to think about it is this is the instantaneous, this is the rate right on the margin at which our cost is changing with respect to quantity. So if I were to produce just another drop, another atom of whatever I'm producing, at what rate is my cost going to increase? And the reason why I'm saying it right on the margin is we see that it's not constant. If our cost function were a line, we would have a constant slope.