Cube-Yo

The obedient cube slides freely along the cord until you tell it to stop. Say go, it goes, say stop, it stops! No one else can make it stop!

Gravity makes the cube slide from the top of the cord to the bottom. But magic makes the free-falling cube stop any where on the cord just by saying stop. To make it move again, just say go and the journey continues. Repeat over and over. When anyone else tries, they can not make it stop.