Limericks are meant to rhyme
Usually, if not all of the time.
But don't be a fool -
It's no hard and fast rule.
Not all of them must.

A cunning and witty young man,
Who hailed from the south of Japan,
Said "The problem with me,
And my limericks, see,
Is that I always try to fit as many words into the last line as I possibly can."