This is a real classic, somewhat more of a puzzle or joke than a trick, though still gets incredible reactions.
The effect is that a small plunger with a hook on the end is insrted into a tube, with a rubber band protruding from the base. The magician shows that the plunger will snap back into the tube. The tube and plunger are then given to the spectators, who despite numerous attempts, cannot make the plunger snap back. The magician can take back the prop, and with seemingly no effort whatsoever, make the plunger snap back into place.
This is a very old idea, but still baffeling to those who do not know the secret.