So if you just draw a straight line from the source of light through the owl, and you just keep going, and you hit the ground, you're going to figure out where the shadow is. So the shadow is going to be right over here. It's going to be right over there. And we need to figure out how quickly is that moving. And it's going to be moving in the leftward direction. So let's set up some variables over here. So let's say, so what's changing?