Steel Ball and Tube

A solid steel ball-bearing is placed on the opening of a metal tube that is too narrow to allow it to pass through. After the usual magical incantations the ball very slowly descends, into the tube, dropping completely out of sight. Then, just as slowly, it rises again, until finally it is resting on the top rim of the tube again. Everything is immediately handed out to a spectator for examination. To their dismay, the ball is too large to fit into the tube, and will only rest on the rim!