Well, we use a little bit of trigonometry right over here. If you took the hypotenuse times the cosine of theta, you would get x. Let me write this right over here. x of t, x of t, is equal to the hypotenuse, 20 meters, that's the length of the ladder, times the cosine, cosine of theta, and I could say the cosine of theta of t just to make it clear that this is a function of time. This comes straight out of trigonometry, actually our basic trigonometric function definitions. Now, why is this useful? Why do I think this is useful?