The One With All The Resolutions - S5-E11
Joey: Do you know something?
Rachel: Do you know something?
Joey: I might know something.
Rachel: I might know something too.
Joey: What's the thing you know?
Rachel: Oh no, I can't tell you until you tell me what you know.
Joey: I can't tell you what I know.
Rachel: Well then I can't tell you what I know.
Joey: Okay, fine. [Pause.] You don't know!
Rachel: All right, how about I go over there and I will walk into Chandler's bedroom and I will see that thing that I think that I know is actually the thing that I think that I know!
Joey: [gasps.] YOU KNOW!
The One Where Ross Can't Flirt - S5-E19
Ross: By the way, if it makes you feel any better, I happen to like 8 year-old boys.
Answer: Usually by the seventh night of Hanukkah you don't know what to get the other person, and you will probably resort to getting the other person something insignificant. Chandler is basically saying that the pen is a stupid gift.