The one where he doesn't cry at the movies and then later on they ask him why he doesn't cry
Joey: Come on, man, there's gotta be something that gets you choked up! Like uh, uh... Oh! What if you saw a three-legged puppy?
Chandler: I'd be sad, sure, but I wouldn't cry.
Joey: Okay, what if the puppy said, "Help me, Chandler. All the other puppies pick on me."
Chandler: Cry? I just found a talking puppy! I'm rich!