Borrow a bill of any denomination from a spectator. Tell them you are going to test it to make sure it is not counterfeit. Pull out the counterfeit detector, and explain to them that you will insert their bill into the machine and identify whether the bill is a fake or not. As you insert the bill into the detector....Oh no!! it is shredding the spectator's bill! Try and calm the spectator down, and then amaze them by returning their bill to them in one piece!!