I say anything is fair game, as long as you can be creative about it. I honestly think, if you approach it correctly, you can make just about anything funny. George Carlin had a very funny bit about rape several years back. Bill Hicks did a bit that started with, "Doesn't Rush Limbaugh strike you as one of those gay guys that likes to lay in a tub while other men pee on him?" that was hilarious. Stomach-turning, but unique and very funny.
As Lenny Bruce once said, "You don't need a government to tell you when you've gone too far. There's no greater deterrent for a comedian than stone silence."
I find it interesting that a guy who wrote a skit about a kid's show host who goes nuts and kills people would ask such a question.
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No2 Vaporizer