|Mysterious Universal Nut|
Effect: The performer displays a double ended bolt and a nut. The bolt is of a different diameter at the two ends, with one end of the bolt twice as thick as the other.
The performer first screws the nut onto the bigger 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.
You may, if you choose, hand the bolt and nut to a spectator to examine, and he will not be able screw the nut onto the broad bolt. The nut is just a perfect fit on the narrow bolt, and there is no way it could have screwed over the broad end of the bolt, except by magic!