The joke towards the end of the opening credits to The Simpsons. Each week (or most weeks) something different happens when the family rush into the living room to watch the TV.
Various examples of the Couch Gag: the couch turns into a monster that swallows the family once they sit on it ... the end wall with the couch retreats down an increasingly long tunnel and they keep chasing after it ... Santa's Little Helper (their dog) is already sitting on the couch and snarls, hair bristling, as they close in ... the family crash into each other and break into little pieces on the floor with a noise like shattering porcelain.
by Fearman August 12, 2007