Faces tell a lot about what a person is thinking or feeling. Unfortunately we don't always read the right message from the face, but faces can lie. We start reading faces as infants looking up into our mother's face, leaning over the crib. The infant learns to read whether she is happy and smiling or angry or afraid. The baby automatically reacts.
Somehow our brains tell us when a smile is a real indication of pleasure and when it is being polite or when it is being deceptive. Sometimes a pleasant smile can hide anger or violence. That is one of the reasons that clowns can be so scarey. We can't tell if they are going to kill us or make us laugh!