Why did the Mona Lisa painting smile?

She was thinking of all the bills she had to pay.

The legend goes that Mona Lisa had bad teeth, so she didn't show them. In those days, dental hygiene was not a personal point, so many people did not have nice smiles. It was also a fact of life that women who bore children did not get enough calcium, so they lost teeth. There is a folk saying, "have a child, lose a tooth" that was common wisdom until the 20th century. So the most common wisdom about Mona Lisa not smiling was that she was a new mother and had lost teeth. It was a very common problem during that time period.