I heard about this when I read Blink by Malcolm Gladwell. I think there was a Radio Lab episode about this, too. Basically, it is almost impossible to hide your true emotions. How you really feel will always show on your face for a fraction of a second, whether you like it or not. It's a pretty useful skill to be able to see these snap expressions to tell what people really feel like when you're talking to them.
Take a test HERE to see how well you do.