A square tube has a threaded hole which allows entry of a screw to pass through completely.
A metal ball, is placed on top of the tube. Obviously it can not pass through the screw. By magic the ball is slowly sinking into the tube to fall on the table! The screw is unscrewed (the spectator can see how it crosses the tube).
This is a great close-up effect and an effect which is almost as wonderful as it works its magic.