Imagine you’re a professional basketball player, about to take a foul shot. You’ve practiced this thousands of times, and you know exactly how you need to set your feet and orient your body in order to get the best results. But then you notice that the floor is uneven, or slippery, or has a big depression right at the foul line, or has a board sticking up under your foot. It’s going to be much harder to make the shot. Well, this doesn’t actually happen on professional basketball courts. But