In the episode entitled "The Cohabitation Experimentation," when Amy’s apartment floods, she proposes a “cohabitation experiment” with Sheldon. Also, Howard and Bernadette are upset when Koothrappali learns the gender of the baby before they do.
Here are the five best lines from the episode:
"You can't have your head shoved in a toilet as much as I did and not pick up a few things about plumbing."
--Sheldon's explanation to Penny and Amy about why he knows so much about her leaking pipes.
"The two of you had sex one whole time. Nothing can put out a fire like that."
--Penny's response to Sheldon claiming that living with his girlfriend could 'ruin the romance'
"It was an accident. The doctor left the folder out on a desk. It's not my fault I opened it and looked."
--Koothrappali explains to Howard and Bernadette how he learned the sex of their baby before they did.
"And no offense, but your bottom radiates enough heat that I'm surprised there aren't iguanas lying on it."
--Sheldon's not sure he's happy sleeping in the same bed with Amy.
"You wouldn't know a confounding variable if two of them hit you in the face at the same time."
--Amy insults Sheldon's scientific knowledge during an argument.