And at time a, we were at s of a position. So the change in position between time a and time b, let me write this down, the change in position between, and this might be obvious to you, but I'll write it down, between times a and b is going to be equal to s of b, this position, s of b minus this position, minus s of a. So nothing earth shattering so far. But now let's think about what happens if we take the derivative of this function right over here. So what happens when we take the derivative of a position as a function of time? So remember, the derivative gives us the slope of the tangent line at any point. So let's say we're looking at a point right over there.