A brass nut is threaded onto a shoestring and your spectator holds both ends of the string – It’s IMPOSSIBLE for you to take away the nut.
You cover the nut with a small handkerchief and reach underneath. After a couple of seconds you remove the nut from the string and hand it to your amazed audience. They inspect the string, the solid brass nut and the shoestring but everything is ungimmicked! Impossible!