An impossible riddle with an unbeatable payoff!
We've done puzzles here on Scam School before, but never have we had one with such an unfair setup, or such a surprising solution.
Setup: In any restaurant with a tablecloth, set a dime between two nickels, and set an upside-down glass balanced on the two nickels.
The Challenge: get the dime out from it's prison without touching the glass or the coins, AND without touching anything with another object.
Note: I like to add the extra red herring of saying they can use "anything on the table" to make it work. That way, they'll spend more time focusing on random objects, and less time thinking outside the box.
The Payoff: using your fingernail, scratch at the cloth in front of the coin, and the dime will slowly "walk" its way out of the glass! It looks like it's moving all on its own, and is a very satisfying payoff the wager.