Episode Titles: Cohabitation

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  The Cohabitation Formulation
  Season 4, Episode 16 - Aired February 17, 2011

  Raj's sister, Priya, is in town again and rekindles her relationship with Leonard, prompting Penny to reconsider her feelings for him. When Bernadette asks Howard to move in with her, he is forced to choose between his girlfriend and his mother.
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  The Cohabitation Experimentation
  Season 10, Episode 4 - Aired October 10, 2016

  After Amy's apartment is flooded, Penny suggests that Amy move into Apartment 4A with Sheldon. Unsure about finally cohabiting with his girlfriend, Sheldon comes around to the idea when he looks at it as a five week experiment in living together. Sheldon suggests they move into Penny's old place, Apartment 4B, so they would know if he's comfortable because he's living with Amy and not just comfortable in his own apartment.

  Meanwhile, Howard and Bernadette decide not to learn the gender of their baby, but they question their decision when Raj reveals that he knows the sex of the baby.