Monica and Rachel answered ten questions in the Friends apartment quiz.
The quiz appears in “The One with the Embryos,” the twelfth episode of season four, first broadcast on January 15, 1998. Ross creates a detailed competition to test how well the two pairs know one another. The categories include personal habits, jobs, pets, and other facts about the friends' lives.
The contest begins as a bet over whether Monica and Rachel know Chandler and Joey better than the men know them. After the teams reach a tied score, the stakes rise: the losing pair must surrender their apartment. Monica and Rachel lose after failing to identify several details, while Chandler and Joey take possession of Monica's apartment. The result is reversed later when Monica and Rachel exploit a technicality involving the apartment's pet rules.
The episode is widely remembered for the quiz and for the phrase “They don't know that we know they know we know.” It is often ranked among Friends' most popular episodes.