Take a long rope; tie it to the bottom of the goal post at one end of a football field. Then run it across the length of the field (120 yards) to a goal post at the other end. Stretch it tightly, and then tie it to the bottom of that goal post, so that it lies flat against the ground. Now suppose that I add just 1 foot of slack to the rope, so that now I can lift it off the ground at the 50-yard line. How high can I lift it up?