If we were to take the derivative with respect to time, so if we were to take the derivative with respect to time of this function s, what are we going to get? Well, we're going to get ds dt, or the rate at which position changes with respect to time, and what's another word for that? The rate at which position changes with respect to time? Well, that's just velocity. So that we could write as velocity as a function of time. Now, what if we were to take the derivative of that with respect to time? So we could either view this as the second derivative, we're taking the derivative not once, but twice of our position function, or you could say that we're taking the derivative with respect to time of our velocity function.