A weightless and perfectly flexible rope is hung over a weightless, frictionless pulley attached to the roof of a building.
At one end is a weight which exactly counterbalances a monkey at the other end.
If the monkey begins to climb, what will happen to the weight - will it remain stationary, will it rise or will it fall?
As the monkey climbs, the weight will rise by the same amount.
Why? As the monkey exerts a force to pull itself up, it increases the tension in the rope, which in turn causes the weight to rise at the same rate as the monkey.