you is in a footrace with the supposed edge of the forums. you allow the supposed edge of the forums a head start of 100 posts. If we suppose that each racer starts running at some constant speed (one very fast and one very slow), then after some finite time, you will have run 100 posts, bringing him to the supposed edge of the forums's starting point. During this time, the supposed edge of the forums has run a much shorter distance, say, 10 posts. It will then take you some further time to run that distance, by which time the supposed edge of the forums will have advanced farther; and then more time still to reach this third point, while the supposed edge of the forums moves ahead. Thus, whenever you reaches somewhere the supposed edge of the forums has been, he still has farther to go. Therefore, because there are an infinite number of points you must reach where the supposed edge of the forums has already been, he can never overtake the supposed edge of the forums