Summarize this article in one sentence.
Ninety percent of human communication is non-verbal. A person's body language can be voluntary or involuntary, and it is both genetic and learned. It is a strong indicator of how a person feels, but it can vary from one culture to the next. Those discussed in this article are indicators of body language in Western cultures. Psychologists have classified six involuntary facial expressions that they consider to be nearly universal across cultures. They are happiness, sadness, surprise, fear, disgust and anger. Each has its own signals or clues, and can reveal a person's feelings. But remember they are often fleeting, and some people mask them well.   Happiness is exhibited by a raising or lowering of the corners of the mouth. Sadness is shown by a lowering of the corners of the mouth and by the raising of the inner portions of one's eyebrows. Surprise can be seen when eyebrows arch, eyes open wide to expose more white area, and when the jaw drops slightly. Fear is revealed through the raising of eyebrows, when eyes open after being shut or narrowed, and when the mouth opens slightly. Disgust is displayed when the upper lip raises, the bridge of the nose wrinkles, and cheeks raise. Anger exhibits itself when eyebrows lower, lips press together firmly, and eyes bulge. Many believe that eyes are the windows to the soul. This belief drove many psychological and cognitive researchers to investigate if  our involuntary eye movements held meaning. Research has shown our eyes make predictable movements when a person is processing a thought or question. The notion that you can tell if someone is lying based upon the direction of his or her eye movement, however, is  a myth. Here's what we do know.  Eye movements in any direction increase when a person is recalling information. Our eye movements stop when something interests us. We also avert our gaze when thinking through something, such as the answer to a question. And our eyes stop moving when we're trying to shut out distractions and focus. Eyes move from left to right (or vice versa) when we are either problem-solving or processing and recalling information. And the harder the problem the more your eyes move.  Eyes blink at a normal rate of 6-8 times per minute. When a person is under stress, that number rises dramatically. Raised eyebrows not only show fear but also genuine interest in a topic. Furrowed eyebrows signal confusion. Researchers say the mouth's movements reveal a great deal about how a person feels. Pursing your lips, for example, is a sign of anger. Happiness, as discussed, is displayed when the corners of the mouth curve up. However, researchers have found that different smiles mean different things.  Natural, spontaneous smiles appear gradually, last briefly and are shown repetitively. Genuine delight is expressed by a series of short smile “bursts” and creases that form at the corners of the eyes. Fake smiles are about 10 times larger than natural, spontaneous ones. They also appear suddenly, last longer than a natural smile, and disappear abruptly. A person tilts his or her head when actively listening to a topic that interests him or her. Nodding your head indicates you are interested in the topic and want the other person to continue talking. And rubbing one's forehead or earlobes indicates a person feels uneasy, nervous or vulnerable. People move their hands and arms more than normal when they are speaking or answering a question. They also touch both things and other people more when they respond to intimate questions or when they are physically close to others.  Hiding your hands, like in your pockets or behind your back, can indicate deceit. Crossing your arms doesn't always indicate anger, but it can be a defensive posture. It can also mean you feel uneasy with the other person. Learning toward another person is a relaxed and interested indicator. Friendly feelings exist. Leaning in too close, on the other hand, can be experienced as a hostile or dominating gesture. Angling yourself toward another person while you're both standing signals respect. It's also often a sign of deference.  Adopting postures similar to others tends to increase group or interpersonal rapport. It tells others you are open to their thoughts. Standing with your legs wide apart is either a traditional stance for those in positions of power or a dominating position.  A slumped posture indicates boredom, alienation or feeling ashamed. An upright posture asserts confidence, but it also can exude hostility or a sense of uprightness.

Summary:
Learn about body language. Understand the six facial expressions. Know what eye movements mean. Look at the way a person's mouth moves. Observe head movements. Watch hand and arm movements. Notice body posture and movements.