How do you read people based on body language?

Personally I am extremely interested in psychology and the general study of human beings. When talking to someone, if they mirror your body language and copy your posture by crossing their legs or leaning inwards this shows they are extremely interested in the conversation you are having. I find the study body language just common sense. A more technical study of body language is if someone is telling you something and they repeatedly look to the left, it could be a lie because they are creating this lie by focusing their attention to left part of their brain which creates images. There is so many parts of body language I could not possibly specify on anything individually because there is so much to say. Maybe narror the question down furthur and then my answer will be so much clearer. Do you want to know about a certain type of body language? I love to be able to answer your question but it honestly depends upon what they are doing, what body language they are doing. All I can really say is you are always emmiting body language, you are always always using. Be more specific and then the answer given will be better.