Even though body language can help you figure out how someone is feeling, your assumptions aren't always accurate. After all, one gesture can mean a million different things. For instance, you might cross your arms when you're angry, or when you're cold, or when you're nervous.
That's why you can't rely on all of the things that you've learned about body language in the past. Some of it just won't apply in most situations.
If you're interested in the science behind body language, then you should watch this video from Mental Floss. It'll tell you everything you need to know about how humans communicate.
What other things do you know about body language?