The fossa is formed by subcutaneous muscle activity. Most muscles of human body are firmly attached to bones through tendon fibers, such as pectoral muscles and lower limb muscles. The exception is the facial expression muscle, which is directly attached to the facial skin. When the expression muscles contract, it affects the facial skin, so there are various wrinkles in this part, which produces expressions of joy, anger, sadness and joy, and can make all kinds of funny facial expressions. There are two dimples on the smiling face, which are formed by the relative influence of facial skin and facial expression muscles such as buccal muscle and laughing muscle.

Not everyone will have dimples on his face when he smiles. Whether there are dimples is related to the development of expression muscles. People with less plump smiling muscles and less subcutaneous fat on their faces generally have no dimples when they smile.