6x18 "The One Where Ross Dates a Student"

Ross starts dating a student from one of his paleontology classes, much to his friends' amusement, although he has fears about how the relationship may jeopardize his career. Rachel's and Phoebe's apartment catches fire, causing them to move in with Monica and Joey. At first it appears that Rachel will get to live with Monica and Phoebe will be forced to live with Joey when it appears that Phoebe's candles caused the fire. However, when it emerges that the fire was actually Rachel's fault, the two switch and Rachel finds herself preferring Joey's easygoing nature while Phoebe chafes over Monica's desire to keep her apartment clean.