Now understand, the words in RED are the base of the joke, and the words in green are basically sick, disgusting improv.
The documentary "The Aristocrats" is about this joke, and it is told by several big comedians.
You can see other (and really sick) versions of this joke
HERE
Or you can check out the website of the movie itself
HERE
I went and saw the movie last night. It was awesome, but you feel really bad for laughing.