You give a child a marshmallow and then tell them you have to leave the room for a bit. If he doesn't eat the marshmallow by the time you get back then he gets to eat TWO marshmallows.
After dinner one night I told William the rules, set the video camera and left the room. Below is his battle with temptation. At first I thought the video was going to be boring, but check out what happens around 1:15. Enjoy!