The joke is simple: the jokester says “Pull my finger” and sticks out his finger, the victim pulls the finger, the jokester farts. My father (still) thinks this is hilarious. As he explains,
“It’s something we did as kids. It was super funny back then. And being a dumb guy, still think it’s funny. Back then we would literally fart when you pulled someone’s finger. Tears of hilarity. So every time I hear it I think of being in grade school.”
My dad “performs” this joke every time I see him. It has become something that everyone in my family expects, and that has become a part of the humor for Dad. It’s basically his way of saying, “I’m going to fart.”
Another part of the humor is that the victim becomes complicit in this nasty thing the jokester is doing. The victim is meant to feel like his action of pulling the finger is what caused the fart. I remember as kids we would be astounded as to how my dad could do that on command. Really he was just fooling us all. This joke is more of a prank; it is most fun for the jokester (unless, of course, the victim respects good toilet humor). Pranks are interesting because they walk the line between being cruel and being amusing. I think they exist so that the prankster can be cruel and get away with it by calling it funny.
Cruelty is a common theme in humor in general. A lot of jokes are deprecating and sometimes mean, but if they’re clever or funny enough, they allow people to find joy in all sorts of things that are usually not very funny. Self-deprecating jokes can be used to brush off embarrassing mistakes. And other jokes can be used to inject a little bit of happiness into depressing events.
Pranks, on the other hand, can sometimes be just plain mean. But I think the “pull my finger” joke is one of the simplest, funniest pranks you can pull on someone… no pun intended.