A German expert said that he had finally found out the "household register" of Mona Lisa, but the mystery of her smile has not yet been solved. Da Vinci's "Mona Lisa" is a riddle that has lasted for hundreds of years: who is this woman with a mysterious smile? In front of this oil painting, which is reputed as the "treasure" of the Paris Louvre Museum, there are always crowds of people. People whisper and guess who this mysterious woman is - whether she is Da Vinci's friend, lover, mother, or even Da Vinci himself?