Free, E.E. Imagine a series of race tracks one inside the other. Imagine these tracks are separated by high board fences. Put a race horse in the outermost track and instruct him to run around it until when he happens to feel like it, he has to jump the inside fence into the next track, run around it for a while, and then jump into the next fence, and so on until he reaches the innermost track of all. If, then you watch this procedure from the ﬁeld outside the outermost fence, you will not see the horse at all as long as he is running in a single track. The fences hide him. But whenever he jumps from one track into the next, you will see him for an instant as he goes over.