Cubio (Brass) by Mr. Magic
This is a nice little trick to amuse your friends.
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, of course. But when you say, "Stop!"...the cube stops in mid air, suspended halfway down the string! It can stop anywhere along the string at a moment's notice. You can also control the speed at which it falls!
Very easy to do, and well made in solid brass.