We were all content with the idea that a cat's purr means, "I'm happy." But a new study suggests that some purrs send cat owners the message that they're hungry. From CBS News:
Researchers found that purrs of hungry cats included a higher-pitched sound, somewhat like a cry or meow. They played recordings of these purrs from 10 cats to 50 human volunteers. Even people who'd never owned a cat found them to be more urgent and less pleasant than contented purrs from the same animals.
Apparently these food-seeking purrs could be exploiting the way humans naturally respond to a baby's cry, the researchers say.

Not all cats use this strategy. But apparently some learn to turn it on when they discover its effectiveness in getting a human to feed them.

[via CBSnews]