We all know a few of them, so let's spread this somewhat under-appreciated art form around a little bit. Compose your own, or provide a little information for others to compose one about you. Feel free to post others' work, if you like, as long as proper attribution is given. I'll start things off with one from one of the masters of Limericism (and the coiner of the phrase which is the title of this topic), Isaac Asimov.
There once was a maid of Peru
Who swore that she never would screw,
except under stress
of forceful duress,
Like "I'm ready, dear. How about you?"
Please note that more obscene does not always mean better, when it comes to Limericks. Test your wits!