It's one of the most hotly debated and heavily discussed subjects on the Internet, maybe even more so than the airplane on a treadmill. Somewhere, on TV, in a movie, in a book, everyone knows they heard a question and answer almost identical to "What does it do?" "Nothing, that's the beauty of it," but nobody can source it. The Simpsons and The Hitchhiker's Guide to the Galaxy are two commonly stated sources (in fact, I reread the entire HHGtG "trilogy" to find the quote and prove it, but came up empty.)
The real source is the TV series Burke's Law, during the episode "Who Killed 711?" and this is the clip to prove it.