Diminishing Bolt by Mr. Magic - Trick

Two giant size bolts are shown. One of the bolts is selected by the spectator. The spectator takes his selection while the magician takes the other one. Now the performer starts to shake the bolt and asks the spectator to do so as well. After some time, it is seen that the bolt held by the performer has diminished considerably and has become very short in length. This can be presented in a mysterious, magical manner, or as a comedy bit - your choice!