This Is Heavy by Jay Mattioli

EFFECT: The performer invites a spectator on stage to help in a demonstration of muscle hypnosis. Seating the spectator on a small wooden stool, the magician "hypnotizes" the spectator into believing that a small silk handkerchief weighs over 1,000 lbs. 

An empty cardboard box is shown on all sides and placed on the wooden stool. A second spectator is now invited on stage and is asked to place the handkerchief in the box and then lift the box with its silk contents off the stool, which proves to be a simple feat. But when the hypnotized spectator attempts to lift the box he can not! It is simply too heavy for him to even budge!

The second spectator can even reach in the box themselves and remove the hanky, which then allows the box to become light enough for the hypnotized spectator to lift off the stool. But whenever the hanky is placed back into box, the hypnotized spectator can not lift the box one bit! This becomes a riot for kid show audiences as well as for adults.