"There once was a man from Nantucket" is the opening line for many limericks. The name of the island of Nantucket creates often ribald rhymes and puns. The limerick is seen as bawdy and obnoxious; it is often called a freak-show curiosity and compared to something that is found in a carnival of literary forms. It has refused to die, despite its curious role as the "vehicle of cultivated, if unrepressed, sexual humor in the English language" (The Horn Book: Studies in Erotic Folklore and Bibliography, 1964, Gershon Legman).

Additionally, it has been noted that the simplicity of the limerick quite possibly accounts for its extreme longevity. It consists of five anapestic lines with the rhyme scheme aabba. The first, second, and fifth lines are trimeter, while the third and fourth are dimeter. Often the third and fourth lines are printed as a single line with internal rhyme. The following example is said to fairly represent the genre (limerick) in both style and tone:

There once was a lady named Cager,

Who as the result of a wager,

Consented to fart

The entire oboe part

Of Mozart's quartet in F-major.

More Info: en.wikipedia.org