Ultra tuerca y perno milagro

Do an easy but surprising trick! A bolt threaded into a nut visibly un-threads itself. The nut and bolt can be totally isolated, in a clear container. The spectator may thread the bolt into the nut. He can examine every thing thoroughly. You can even have the effect work on a spectator's hand.

No elastic bands, no exchanges. This mysterious effect is as close to real magic as you can get.