Salto Mortale

A short and a long rope, each brightly decorated, are presented to the audience for selection. A member of the audience chooses one of the ropes and is allowed to tie a silk to it. He may tie a double knot if he wants to. Let us assume he has tied the silk to the short rope. You hold the long rope next to the short one, move the two ropes apart again, and what do you think has happened? The silk tied to the rope has jumped from the short rope to the long one. Once again you hold the two ropes next to each other, and again the silk jumps back to the short rope. Then back to the long one, the short one .... and finally - or in the middle of it the member of the audience is allowed to check whether his knot still as firmly tied as it was at the start.