Definition from Wikipedia: When asked at the end of a series of other riddles, whose answers are clever, obscure, and tricky, this answer's obviousness and straight-forwardness becomes part of the humor. 
Besides the humour, there is a philosophical touch to this question, as one could argue "Did the chicken really cross the road?" Does the road exist for it? I am sure a chicken would not subconsciously be aware of crossing a 'road'. A man-made contrivance like road makes sense for a human rather than a chicken. It could also raise pertinent questions on the consciousness of an animal. Are chickens self-aware of the actions they perform -like crossing the road? More of that later because I want to write about something else today.
The art behind one-liners
What makes this question and the subsequent answer funny? Edward De Bono in his popular book Lateral Thinking writes:
"...there is often an alternative way of arranging available information. This means that there can be a switch over to another arrangement. Usually this switch over is sudden. If the switch over is temporary it gives rise to humour....the reaction to an insight solution is often laughter even when there is nothing funny about the solution itself.
A policeman was seen walking along the main street pulling a piece of string. Do you know why he was pulling the piece of string?.... Have you ever tried pushing a piece of string?
In this situation an expectation is generated by the way the information is put together. Then suddenly this expectation is thwarted but at once one sees that unexpected development is another way of putting things together."
In other words, it is the unexpected response to a build up that makes one find it humorous. Comedians often use this art on stage, where they weave a story around a joke and the end is something unexpected, a different or innovative solution to the original problem. Something that makes the audience think "Oh, I didn't think he would do/say that!". The realisation of the 'Oh-factor' is what makes the joke funny.
How mind works?
The best way of comprehending this is to understand the way our mind processes information.
Sample this:
"Borrow money from a pessimist
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.
.
.
.
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they don't expect it back!"
As you read through the sentence above and finish the first part, the brain identifies a problem it needs to find a solution to. "Why should one borrow money from a pessimist? Why only money? What could/should one possibly borrow from a pessimist? How does a pessimist come into picture here?". The mind races for possible solutions to a jumble of questions like these.
And then, the answer is revealed. Its a twist at the tail! There is a low probability that an average human would guess that answer. Unless of course if you have read it before or you are a genius. As Information Theory goes, the amount of information present in an event is inversely proportional to the probability of its occurrence. For example, if sun rose from the west tomorrow morning, it would make all the newspapers. It is the low probability of the answer that flummoxes the reader with a lot of information. The mind recognises it as a stroke of brilliance, but since it concerns with a trivial matter, the spark is temporary and leads to humour.
Individual perspective to the question - funny!
I will leave you now with some funny answers to one of the most (along with Lightbulb joke) humorous riddles of all time - Why did the chicken cross the road?
Aristotle:
To actualize its potential.
The Dead Sea Scrolls:
And God came down from the heavens, and He said unto the chicken, "Thou shalt cross the road." And the Chicken crossed the road, and there was much rejoicing.
Albert Einstein:
Whether the chicken crossed the road or the road crossed the chicken depends upon your frame of reference.
Freud:
The fact that you thought that the chicken crossed the road reveals your underlying sexual insecurity.
Bill Gates:
To purchase Chicken 2.01a, which will both cross roads and calculate the energy it used. There are bugs, yes, but if you uninstall Traffic 2.0 and Farmer 1.2 it will run. If it freezes at WhiteLine 2.0, we have a patch ...
Grandpa:
In my day, we didn't ask why the chicken crossed the road. Someone told us that the chicken had crossed the road, and that was good enough for us.
Immanuel Kant:
The chicken, being an autonomous being, chose to cross the road of his own free will.
Martin Luther King, Jr.:
I envision a world where all chickens will be free to cross roads without having their motives called into question.
Jack Nicholson:
'Cause it ***** wanted to. That's the ****** reason.
Saddam:
I don't care. Catch it. I need its eggs to make my omelet.
You can read more funny answers here.
Image Credits: Wikipedia
