First of all, I give you the setup and the punchline, there's no need to type those into the form. They're already there.
Some people would like to change the setup, because maybe they want to use different family members or what not. Don't worry about it. Simply have anyone else you might need come in the agent's door when you need them.
Most important, be as inventive as possible. Submit your version of the joke, make it as unique as possible. As they say in the film, it's the singer not the song.
If you aren't familiar with the Aristocrats joke, see the movie! Failing that, here are a couple of links which show the joke in action. They'll give you a basic idea of how to construct the joke. As a warning these links are Not Safe For Work. (But if you're already on this site, you're probably not too worried about your job, right?)
South Park Version This is from the upcoming Aristocrats documentary produced by Paul Provenza and Penn Jillette.
Trey Parker Tell It (MP3)
The one features South Park co-creator Trey Parker telling a different version of the joke in mixed company.
More on the Joke
(All links open in a new window)
The official site for The Aristocrats documentary.
The Internet Movie Database page for the Aristocrats movie
Gilbert Gottfried at the Hugh Hefner Roast (Google cache)
New York TImes columnist Frank Rich on the joke