For a ball to roll down a plank of wood 36" long, it has to cross the half way point at 18". From that point to reach the end, again it has to cross the next half way point at 9", from that point to the end, the ball has to cross the next half way point 4.5" - a number can always be divided in half so the ball never reaches the end - it may look like it is close to, but always has to cross the half way point first.

Don't lose sight of those half way points.