Why does the Mona Lisa's smile vanish when you look straight at it?

The smile seems to flicker because Leonardo refused to draw it with a line. He painted the face in sfumato, from the Italian for "smoked off": a technique for softening the transition between colours, imitating the way the eye blurs whatever it is not focusing on. Leonardo described it as working without lines or borders, "in the manner of smoke". Because he did not draw outlines, the Mona Lisa appears alive to an unusual extent, and the softest blending sits in two places: the corners of the mouth and the corners of the eyes.

That is where your eyes come in. Margaret Livingstone of Harvard reported in 2003 that the smile disappears when you look straight at it, using direct, foveal vision. The human eye is less suited to pick up shadows directly, while peripheral vision picks them up well. On her account, the shadows at the corners of the mouth carry the smile, so when you look away from the mouth the smile appears, and when you stare at it the shadow slips away.

Sfumato was not Leonardo's secret alone. The art historian Marcia B. Hall counts it as one of four modes of painting colours available to High Renaissance painters, and Correggio, Raphael and Giorgione also used it. The painting itself has changed since: an 1809 cleaning removed some of the uppermost paint layer, so what we see is not quite what Leonardo left.