She is widely seen as proof that good looks can last for ever. But, at nearly 500 years of age, time is catching up with the Mona Lisa.

　　The health of the famous picture, painted by Leonardo Da Vinci in 1505, is getting worse by the year, according to the Louvre Museum where it is housed.

　　"The thin, wooden panel on which the Mona Lisa is painted in oil has changed shape" the museum said. Visitors have noticed changes but repairing the world's most famous painting is not easy. Experts are not sure about the materials the ltalian artist used and their current chemical state.

　　Nearly 6 million people go to see the Mona Lisa every year, many attracted by the mystery of her smile. "It is very interesting that when you're not looking at her, she seems to be smiling, and then you look at her and she stops，" said Professor Margaret Livingstone of Harvard University. "It's because direct vision (视觉) is excellent at picking up detail, but less suited to looking at shadows. Da Vinci painted the smile in shadows."

　　In 1911, the painting was stolen from the Louvre. During World War ll, France hid the painting in small towns to keep it out of the hands of German forces.