It's a shadowy smile that has obsessed, even mocked centuries of art lovers with its elusive hint of pleasure — and has inspired at least one pop song. But now Philippe Walter and his team of researchers at the Center for Research and Restoration of the Museums of France say they have found the key to the smirk that distinguishes Leonardo Da Vinci's masterpiece, Mona Lisa. Here, a quick summary of their findings, as published in the journal Angewandle Chemie:

What's the secret behind the smile?
Though Da Vinci is known as a master of sfumato — a Renaissance technique that layers light and dark translucent paint to create an illusion of smoke — the particularly subtle method he applied to Mona's mouth has evaded scholars. As the Sunday Telegraph reports, Walter and his team have concluded that Da Vinci painstakingly used his finger to apply 40 or so super-thin layers of glaze — each up to 50 times thinner than a human hair — to create "the slight blurring and shadows around the mouth that give the Mona Lisa her barely noticeable smile that seems to disappear when looked at directly."

How did we finally figure this out?
Technology. Previously, scientists would have to take physical samples from the painting — and France was understandably reluctant to partially deface the world's most famous portrait. Walter and his team, however, were able to use X-ray florescence spectroscopy to analyze the painting's layers in great detail without touching it.

What does this tell us about the Mona Lisa?
It helps explain why Da Vinci took at least four years to paint her; each of the layers of glaze would have required weeks or months to dry. According to his 16th century biographer Giorgio Vasari, Leonard never considered the painting finished.

Are there other unsolved mysteries?
Yes. Chiefly, Mona Lisa's identity. Most art historians name Lisa Gherardini, the wife of Florentine silk merchant Francesco del Giacondo (a presumably faithful wife, since Da Vinci placed her right hand over her left, a coded gesture of the time). Nevertheless, some believe it is a portrait of Leonardo himself.

Will her smile lose its enigma in light of this discovery?
Not likely, says Walter. Merely demystifying Da Vinci's technique doesn't lessen his achievement: "Even today, Leonardo's realization of such thin layers still remains an amazing feat."

Sources: Sunday Telegraph, Art Knowledge NewsWikipedia, Herald Sun, Greatenjoy