Cubio Brass

This little effect has been around in many different incarnations over the years. It's fun and fooling.

You show a small brass cube threaded on a string, and when you hold the string in your hand and stretch it out vertically the cube slides down. 

If you reverse your hands it will go the other way...
Magically you can make the cube move up and down, stop it in place, and control the speed all at your command.