Right at this moment, it's equal to 15. But it is actually changing. And let's call the distance between the shadow and the mouse x. Now, given this setup, can we come up with a relationship between x and y? And then using that relationship, what we're really trying to come up with is what is the rate at which x is changing with respect to time? We know what y is right at this moment. We know what dy dt is right at this moment.