Pull my finger is a joke or prank regarding flatulence in which a mark is asked to pull the finger of the joker, who simultaneously expresses gas (aka farts) in an unsavoury fashion so as to suggest a causal relationship between the pulling of the finger and the subsequent expulsion of gas.
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.
A variation on this joke appears in Yasujiro Ozu's film Good Morning (1959). Schoolboys ask each other to push their foreheads, responding with an expulsion of gas.
In 2008, an iPhone app called Pull My Finger was one of the most popular apps in Apple's App Store, purchased over 50,000 times in less than one week. It allowed users to pull a virtual finger, activating the sound. The phrase is now the focus of a legal battle between Pull My Finger and iFlatulence over the use of the phrase.