Crazy Cube

The magician hands the spectator a cube and two boxes. When your back is turned, the spectator picks a number on the dice and puts it in the smaller of the two boxes. This box is then locked in the larger box. Despite the fairness of the procedure, you can always announce which number the spectator has chosen.

It is virtually self-operating. Repeat immediately. Any number can be freely chosen. No constraints. Everything can be tested before and after.