A true classic pocket trick! The solid steel ball is too large to fit inside the steel tube, but at your command, it lowers into the tube and comes back up again. When your spectator tries it, the ball is now too small for the tube and falls right through it!
 
For a version of this pocket illusion that works on a different principle, see the Brass Ball and Tube. And to learn some great tricks you can do with the Steel Ball and Tube, see the Steel Ball and Tube Book.