Tie an 18-inch piece of string so there is a loose, open, overhand knot in the center. Now tie the ends together with several knots so the string makes a loop. Let someone examine the string and explain that you are going to remove the overhand knot without untying the other knots. Place the string behind your back and in an instant produce it again showing that the overhand knot has disappeared from the loop.
Solution: While the string is behind your back, open the overhand knot out wide, and move it up and lin all the other knots so it will not be noticeable.