Mona Lisa's smile explained by technique

Over the centuries, thousands of people have pondered the mystery of Mona Lisa's smile. Now French researchers believe they can explain the enigmatic expression: it was da Vinci's technique.

“Light and shade should blend without lines or borders, in the manner of smoke," Leonardo wrote. According to the researchers, this was achieved by application of up to 40 layers of ultra-thin glaze that created "subtle optical effects that blur outlines and blend shadows as with fog."