If you don't know what they are, they're a 5-line poem that rhymes like this: (aabba). They are often used in comedy. Here is an exampe of the top 7 (it's also my favourite):

There was a maid from Japan,

Whose limericks never would scan.

When they asked her why,

She said "because I

Like to squeeze as many syllables into the concluding line of the limerick as I possibly can"

Now you write one.