Season 9: Episode 204: The One with Phoebe's Rats
Ross and Rachel have a new nanny (Molly), who is hot. Ross tells Joey not to go after her, which just makes him want her more. Mike discovers that Phoebe has a rat in her apartment, which she treats as a pet (or rather, as an occasional visitor for whom she puts out food). Then they discover that the rat has had babies. Mike mistakenly kills the rat, and Phoebe feels they are responsible to raise the orphans. Rachel continues to fight with her co-worker, Gavin; Monica invites him to Rachel's birthday party. Gavin comes over at the end of the party and makes a move on Rachel; they kiss. Ross is upset when he sees them his apartment, as he's cleaning up after the baby. Molly turns out to be gay.