This is a classic that has stood the test of time! Show your spectator a device whereby you insert a plunger into a tube and catch a rubber band on the end of the plunger's notch. You then SNAP the plunger several times, showing that it's caught the rubber band perfectly.
Hand it to the spectator - they cannot do it! For the life of them, they try, they twist it, they do everything except snap the snapper. They hand it back to you and you can do it instantly!