Her enigmatic expression has been the topic of artistic debate for hundreds of years. But the reason the Mona Lisa’s mouth — part smile, part pursed lip — is so confounding has to do with the eyes, according to one Harvard scientist.

More specifically, Leonardo da Vinci’s 16th-century masterpiece beguiles observers because of the way their gaze jumps around the picture — from the Mona Lisa’s mouth, to her eyes, to her forehead. Where a person focuses his or her eyes determines the extent of the subject’s smile, said Margaret Livingstone during a recent talk.