We shouldn't let National Poetry month go by without considering the limerick. You can find the Library's limerick books by clicking here. There are also lots of very funny ones to be found online at the Limerick Database. Not all of them are good, and not all of them are clean - you have been warned! - but here's an example of one that's both. (Bonus: it's not just a limerick, it's a template for would-be poets who are metrically challenged!)
There once was a [person] from [place]
Whose [body part] was [special case]
When [event] would occur
It would cause [him or her]
To violate [law of time/space.]
Heh. Thank you, anonymous poet!