What makes certain expressions universal? And why are they expressed in these particular ways? Scientists have a lot of theories rooted in our evolutionary history. One is that certain expressions are important for survival. Fear and surprise could signal to others an immediate danger. Studies of humans and some other primates have found that we pay more attention to faces that signal threats over neutral faces, particularly when we're already on high alert. Expressions also could help improve group fitness by communicating our internal states to those around us. Sadness, for example, signals to the group that something's wrong. There' s some evidence that expressions might be even more directly linked to our physiology. The fear expression, for instance, could directly improve survival in potentially dangerous situations by letting our eyes absorb more light and our lungs take in more air, preparing us to fight or flee. There's still much research to be done in understanding emotional expression, particularly as we learn more about the inner workings of the brain. But if you ever find yourself among strangers in a strange land, a friendly smile could go a long way.