18. Pork Is Not A Verb (Season 11, Episode 2 - Brother's Little Helper)
Here's one of the most outright crude chalkboard gags in the entire history of the show, the irony being that "to pork" is slang language for having sex with someone. Boys will be boys, but even so, this was a pretty cheeky nod on the part of the show's writers, all the more so considering that Fox is a traditionally conservative network which likes to keep kids and discussions of sex very much separate. At least it's obscure enough that kids probably won't get it and invoke the ire of their parents, though.