Performer displays a cylindrical brass block, and a brass tube closed at one end. The block will slide easily into the tube

The block and tube both have a hole that will allow a metal rod to pass thru.

When the block is placed in the tube, and the rod pushed through the hole, the block is locked in place inside the tube. It cannot be removed. Yet the performer causes the block to penetrate the metal rod with a mere tap of his hands.