My three favourites:

I sat next to the Duchess at tea
And she said, "Do you fart when you pee?"
I replied, with some wit,
"Do you belch when you shit?"
And thought it was one up to me.

There was a young lawyer named Rex
Who had a small organ of sex.
When arraigned for exposure,
He said, with composure,
"De minimus non curat lex."

(The law does not concern itself with small things)

There was a young man from Cape Horn
Who wished that he'd never been born
And he wouldn't have been,
If his father had seen
That the end of the rubber was torn.