When people greet each other they move their eyebrows up and down -- .

A child must learn some actions, but other actions are innate. All humans are born with the same innate actions. One innate action is sucking. All babies know how to suck when they are born. Babies do not have to learn how to suck. Smiling and frowning are also innate actions. All babies smile and frown, even babies who are blind and deaf. Likewise, all babies cry, although babies who are deaf cannot hear themselves cry. When people greet each other, they move their eyebrows up and down rapidly. Anthropologists believe that this movement of the eyebrow is innate, but all people that anthropologists have studied raise and lower their eyebrows when they greet each other. While anthropologist know that babies' actions are innate, they do not know if some adult actions are innate or not. Adults have learned many actions and there is no way to be certain that an adult action is innate.