You show an elegant velvet bag with three colored ropes dangling from it. You say that one rope is different from the others (colors aside). A spectator FREELY (and we mean FREELY) selects one rope and HE removes the other two from the bag. They do not have a knot. He then removes the third rope and it is seen to have a knot (and the spectator can UNTIE it!).
Comes complete with ropes, bag, and detailed instructions for performing this awesome effect!