Cut No Cut Scissor
The performer, holding a rope, hands over a pair of scissors to the spectator and asks her to cut the rope in the middle. The spectator fails to open the scissors. She exerts her full strength but cannot separate the blades. The performer takes the scissors and immediately opens the scissors without any extra effort. He now hands over the scissors, with the blades separated, to the spectator. But now she cannot cut the rope as she fails to close the scissors! Finally, the performer takes the scissors and cuts the rope.
This is done in a fun, comedic manner, so the spectator joins in the laughter.