Four pieces of card stock, each a different color is placed on a table in front of a spectator. The spectator is asked to turn one of the colored cards either over or 180 degrees around.
This is done while your back is turned. When you turn back you pick up all the cards and instantly tell the spectator which card he turned around. To prove that the cards are not marked in any way the spectator is asked to put each card in a small envelope and then turn one of them around. Once again you will be able to tell which envelope was moved.
