Scientists theorize that eyebrows help divert stinging sweat and vision-obscuring moisture such as rain from running into the eyes. The arch shape helps to keep liquids to the sides of the face. Eyebrows also play a significant role in nonverbal communication.
There are a variety of eyebrow motions; raising both eyebrows simultaneously generally expresses surprise or excitement. Raising one eyebrow but not the other is typically interpreted as a questioning look. Eyebrows that are drawn together indicate an expression of concentration or intensity.
It is considered fashionable in some cultures to maintain eyebrows, like other body hair. Other cultures consider it attractive to let eyebrows grow together, forming what is called a "unibrow."