How many points are there on the globe where, by walking one mile south, then one mile east and then one mile north, you would reach the place where you started?
Here is what you have to do. You have to throw a ball as hard as you can but it must return back to you even if it does not bounce at anything. Also, you have nothing attached to the ball. There is no one on the other end to catch that ball and throw it back at you.