A limerick is a form of verse, usually humorous and frequently rude, in five-line, predominantly anapestic meter with a strict rhyme scheme of AABBA, in which the first, second and fifth line rhyme, while the third and fourth lines are shorter and share a different rhyme. The following example is a limerick of unknown origin:
The limerick packs laughs anatomical
Into space that is quite economical.
But the good ones I've seen
So seldom are clean
And the clean ones so seldom are comical.