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"Why did the chicken cross the road?" is a common riddle or joke in several languages. The answer or punchline is: "To get to the other side." The riddle is an example of anti-humor, in that the curious setup of the joke leads the listener to expect a traditional punchline, but they are instead given a simple statement of fact. "Why did the chicken cross the road?" has become largely iconic as an exemplary generic joke to which most people know the answer, and has been repeated and changed numerous times.
The riddle was mentioned in print in 1847, in The Knickerbocker, a New York City monthly magazine:
...There are 'quips and quillets' which seem actual conundrums, but yet are none. Of such is this: 'Why does a chicken cross the street? Are you 'out of town?' Do you 'give it up?' Well, then: 'Because it wants to get on the other side!'
The joke had become widespread by the 1890s, when a variant version appeared in the magazine Potter's American Monthly:
Why should not a chicken cross the road?
It would be a fowl proceeding.

http://en.wikipedia.org/wiki/Why_did_the_chicken_cross_the_road%3F