Mysterious Universal Nut
One of the best magic puzzles you can carry in your pocket.
This one is a favorite of mine. Show a Nut and a double-ended bold. Each end of the bolt is a different size. The nut seemingly is only the smaller size. Show the nut goes on the small side then put it on the large side and the nut screws on easily. Take the nut off and show the small side again. Hand out the trick for examination.