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You show, a wooden block, that has a hole in one end, and the ends of a rubber band sticking out at the other. Into the hole you place a plunger with a hook on it and show how easy it is to hook it onto the rubber band. When you hand it to a spectator, they just can't get it to hook on.
This is a classic effect, made in hand turned wood finish, these are the best you can get!