The classic related rates problem. You might call it "the shadow problem" or you might know it as "the lamp post problem" or maybe just "the light problem." Guy (or girl) of a certain height walks away from a street light at a certain rate. You then have to find two different things: the rate at which his shadow is growing and the rate at which the tip of the shadow is moving away from the light. Lots of people think these are the same thing at the outset, but they're not!
