You show a key chain with a metal disc in the key chain. You hand out the key chain and ask the audience to remove the disc from the key chain. They may try as much as they want but they would not be able to remove the disc from the key chain, but you can remove it in a second. There is only one unique way to remove this disc from the key chain.
You can use a quarter or a nickel instead of the metal disc to make it more challenging.