In a similar vein, tell your victim that you have a test of coordination
you would like him to try with you.  Find a door with a fairly large crack
between the door and the wall when the door is open.  (You know, over by
the hinges; across the width of the door from the doorknob...)  You need an
egg (NOT hard-boiled), and a wood floor (you don't want the egg cracking on
carpeting, do you??).  Now, have your victim get on the opposite side of
the door from you, and put 2 fingers through the door.  Hand him the egg,
in those two fingers.  Working with him, trade the egg back and forth a
couple of times, moving UP the door frame.  After you have his confidence,
leave.  He will be trapped there, holding this egg by two fingers through
the door.  If he lets go and nobody takes the egg, it will crash to the
floor.  Best to do in the person's own room.