You show a decorative key that can be freely inspected by the audience. You then borrow a ring from the audience and put it on the shaft of the key. You then give it back to the audience stating that if they can take the ring of the key they can get it back or else you get to keep it.
Try as hard as they want, the audience cannot remove the ring from the key but you can remove it in a jiffy.
Both the ring and the key can be inspected at all times. There is nothing to add or remove to the key at any time. Everything is self contained and works automatically.