Banknote prison

You show two crystal clear Plexiglass plates between which a bill (EUR) is held in place by two rubber bands. Then let a spectator the bill with a lock whose strap is passed through the holes located in the plates, and thus through the bill, secure. Then, the audience must sign the bill as secured by an attached only in the upper plate hole. She herself also identify a note you stick together with the trapped between the plates under a cloth. After a moment, you bring the hand back under the blanket out and the bills have swapped places. Then you can open the lock and examine both plates and the banknote. Magic Items are non-returnable.