The reason why the painting is so famous is that it seems to be full of mystery. Mona Lisa's eyes look out at the viewer. But no-one can guess what she is thinking. Her eyes and her mouth seem to be smiling. This is very unusual in a portrait painting. Most people in portraits look very serious. It is hard to tell what Mona Lisa's exact expression is. When a person wants to read another person's feelings, they look at the corners of their mouth and eyes. But Leonardo has painted soft shadows in the corners of Mona Lisa's mouth and eyes, to disguise her expression. The soft shadows are also found on the sides of her face, her neck and hands. The way that Leonardo uses shadow is called "sfumato" (which is an Italian word for "smoke"). Vasari said that the picture was so beautifully painted that every other artist who looked at it thought that they could never paint so well.