Cats are basically bastards that exploit us by purring annoyingly when they want us to do something, according to researchers. Apparantly, when they're trying to get food, a cat's purr contains more high-frequency, cry-like sounds, which make it irritating. The paper's in Current Biology.
My cat has yet to master this technique - rather than embedding an annoying cry in his purr, he just meows annoyingly. And if you try to walk away from the food bowl without feeding him, he bites your feet.