Robert K Foster15 Jan 2003

Limericks use the distinctive rhyming scheme of a-a-b-b-a, where the first two lines set the scene, the third and fourth lines quickly twist things about, and the fifth line hammers the conclusion. (first, second, and fifth lines are trimeter, third and fourth are dimeter).
this is from the help screen, not my original writing.
Limericks focus on the absurd or the silly when they're not downright naughty.