One of the best puzzles you can carry in your pocket.
The performer displays a double-ended bolt and a nut. The diameter at one end of the bolt is twice the thickness of the opposite side. The performer first screws the nut on the bigger end of the bolt, and it fits perfectly. He then unscrews this, and proceeds to screw the nut on the narrow end of the bolt, and once again it fits perfectly. Mechanical prop, incredible effect.