Season 6: Episode 135: The One Where Chandler Can't Cry
Joey, Phoebe and Monica discover that Chandler can't cry no matter what the circumstance. Joey tells him that he's dead inside and Monica makes repeated attempts to elicit some emotion from Chandler. Rachel asks Ross to stop seeing her sister, Jill. Ross agrees to cancel their date, but when he tells Jill she doesn't take it very well. Rachel admits to Jill that it was she who asked Ross to stop seeing her because of their "history" together. Jill gets mad at Rachel for trying to control her life, so she goes over to Ross's apartment to use him get back at Rachel. Later, when Ross tells Rachel that Jill kissed him, Rachel is furious. He then admits that he stopped the kiss because he didn't want to ruin the chance of them getting back together someday. After witnessing this encounter, Chandler is finally able to cry.