Body language experts are trained to look for groups of behaviors. Often, certain behaviors such as crossed arms could just mean the subject is cold. It is also hard to judge someone's body language after you have just met them; it is much better to compare their behavior to the "baseline" of their normal behavior. That is because certain behaviors are not innate but learned unconsciously from parents, relatives, close friends, and is also dependent upon regional culture. If you want to learn about body language, you should spend a lot of time practicing watching several different people and finding groups of behaviors that signal their respective moods before you start to start to make inferences with confidence. It is helpful to watch people from different cultures or different classes so that you may see how much variation there is. Otherwise, decisions made with little knowledge and practice is actually harmful because your inferences will likely be wrong. "You should never cross your arms again" may be good advice, but only because you can then guard yourself from people that overestimate their powers of observation.