Q:

Why is "pull my finger" a joke?

I've heard of this joke in the TV show "Friends" several times, and I don't understand why it's funny.
Could someone give me a hint?
It's hard to get Chandler's joke for non-English speakers.

A:

It's a childish joke.
Kid1 asks kid2 to pull their finger. When they do kid1 farts - as if pulling the finger, like pulling a lever, had caused the fart to be released.
It's an Americanism, or at least I never heard it in BE growing up.
Earliest reference I can find is "The Street" (1969) by Canadian author Mordecai Richler:    

“ He settled in sullenly at the kitchen table, his smile morose, and
  suddenly he would call out, "Pull my finger!" If you did he let out a
  tremendous burp. ”

But you have to assume that it's older than that, my guess would be dating back to steam engines where you had lots of levers which blew off clouds of steam when you pulled them.

