Mona Lisa (1503-1506, Louvre, Paris), Leonardo da Vinci¡¯s world-famous portrait, was the artist¡¯s favorite painting; in fact, it went everywhere with him. Although there have been many theories about the origin of the inexplicable smile on the woman¡¯s face, it was probably just the result of Leonardo¡¯s interest in natural chiaroscuro (the effect of light and shadow on the subject).