The One With The Embryos (Season 4, Episode 12)

Phoebe accepts to be a surrogate mother as everyone else engages in a fun quiz game. Monica Geller's competitiveness should never be underestimated. She wouldn't let go of the football from her brother Ross in the preceding Thanksgiving episode, and she's not going to back down from some fun wagers with the lads - despite Rachel's reservations. Chandler and Joey, on the other hand, have a lot to lose with their pet duck and their chick-turned-rooster.

They also have a lot to gain because Monica has agreed to let them take the girls' flat if they win. None of this is meant to detract from the far more serious nature of Phoebe's story, which has plenty of silly elements of its own, but it's all quite memorable nonetheless.