Super Obedient Cube*

A cube threaded on a pair of strings falls freely, when the string is held vertically. But it stops whenever the performer or a spectator commands it to "STOP", then falls again when commanded to "GO" But then the performer calls "UP" and the cube crawls up the string, defying the law of gravity and the performer can control the cube to STOP, GO, or UP as he wishes. Easy to do, ready to "GO" !