Try to tickle yourself. The center of your right foot is the most ticklish part of the body.

You can't do it. Even if you try to tickle yourself in the exact same place and in the exact same way someone else tickled, you will not feel anything. You don't laugh hysterically, as you would if someone else tickled you, despite the fact that the sensations sent through your nervous system to the brain are exactly the same. There is no biological reason for tickling and the ensuing hilarity. Yet in the brain, there obviously is something that triggers laughter when we are tickled, when something unexpected happens, or when someone tells a joke.

Why was six afraid of seven?
Because seven ate [eight] nine.

Most likely, this joke is so overdone that it isn't even funny to you. However, if you told someone around the ages of four or five, they would laugh. The brains of younger people find this funny, triggering laughter. According to one theory of laughter, this is because their brains haven't associated the setup line "Why was six afraid of seven?" with the punch line "Because seven ate nine". This theory is called the incongruity theory.

Our brains are very successful in completing what evolution has programmed them to do: think. Brains are so good at thinking that you involuntarily predict what will happen as a result of your current state. For example, when your friend walks up to you and greets you, you usually don’t have to think, “Open your mouth. Let your vocal cords stretch and vibrate. Use your tongue to manipulate that sound into an intelligible word.” Your eyes would simply see your friend; the upper right part of your frontal lobe would remember his or her face; and the word hi would spontaneously come out, as if your brain predicted this exact scenario.

This predicting function of the brain does not stop when a joke is told. When you hear “What did one eye say to the other?” your brain immediately, without your knowledge, begins to think up logical answers. Your brain treats this joke set up question as an actual, answerable question. Hello. I was not aware we could talk to each other. I don’t think that eyes can talk.
This cold, rational logic is broken by the introduction of the punch line of this joke: “Between you and me, something smells”.

This is funny because it fits into what the incongruity theory defines as funny. This joke mixes the mundane (the placement of the nose and the eyes on the face) with an unexpected pun (smells as both an adjective and a verb). Jokes work off this feeling of surprise. This joke began with one narrative, the dialogue between two eyes, and ended with a completely separate focus.
Neurologically, the brain began to formulate and emotionally invest in a narrative when the set up line was told, but had to abandon that narrative when the separate focus of the punch line was introduced. The brain had to do a total flip-flop. The stress caused by this turnaround is followed by a rush of different emotions. When we hear a joke, we hold two sets of incompatible data in our brains almost simultaneously. Our bodies’ way of relieving the stress of this is through laughter.

To return to the original point, the incongruity theory states that once the seven-eight-nine joke is told, once you realize that seven ate nine, the joke falls flat on its face. This joke will never be funny again. The only way to rejuvenate this joke is to twist it so that it is the complete opposite of what it desires to be: an anti-joke.

Anti-jokes are jokes that take frequently used set up lines and anticlimactically match them with no punch line. The reason anti-jokes are funny is because you expect a joke, but you receive a logical answer. Anti-jokes are surprising in their lack of surprise, fulfilling the objective of the incongruity theory. This makes an anti-joke funny, despite being completely and predictably logical.

Why was six afraid of seven?
Numbers are not sentient and therefore cannot hold human emotions such as fear.

Jokes and anti-jokes make us hold our conflicting notions of what “should happen” and what “happens”. For this reason, humor has always been a tool to aid in understanding conflicts. Jokes and comedy exist because there is no better way to understand the absurdity of life than to laugh at it. Jokes challenge our belief system, showing us that there is no clear-cut ending.

So go ahead. Tell that fart joke. It advances humanity’s understanding of the world’s most complex structure: the human brain.