Different emotions are expressed in different ways, and each emotion usually has a readable physical reaction. Using these, you can get clues on how a person is feeling, whether it's towards you, somebody else, or how they feel about the world at the moment. For example, when somebody is happy, you usually see a grin on their face, they may skip around (if they are extremely happy), or treat everybody else noticibly better. However, if a person is sad, they sometimes have a frown on their face, they walk around rather slowly, and sometimes react badly when you try to talk to them. It is these reactions that clue a person in on how they are feeling, and then how to treat them as a result. In this youtube clip, notice how one person's mood can affect everybody they come in contact with:
http://www.youtube.com/watch?v=o4msnQ4KiVk
I like how you incorporated the video in your blog. I would have liked more information on the topic, such as an example of one of the studies representing how emotion is displayed physically.