As noted by the audience, the rest of the Friends, and even the man himself, Chandler makes a lot of jokes. It’s his thing. Of course, most of them are either genuinely hilarious or so bad they’re good. But occasionally there’ll be one that just flies right over your head.
Perhaps the best example of this comes in The One With All The Poker, as Rachel excitedly tells the guys to guess her good news (that she has an interview at Sak’s Fifth Avenue). Chandler responds in a typically Chandler way, with a joke, quipping “The fifth dentist caved, and now they’re all recommending Trident?” Erm, good one?
Trident is a brand of gum, and back in the 70s their parent company did some market research, with the results being that 85% of dentists surveyed said they recommended sugarless gum. They published the results as saying 4 out of 5 dentists recommend it. As obscure jokes go, this is one that is definitely difficult to spot.