Why is the Mona Lisa so famous?

There's a small painting in Paris of a woman with a faint, mysterious smile. People travel across oceans to stand in a crowded room and squint at her through a sheet of glass. She isn't very big — about the size of a poster you'd tape to a wall. So here's the real puzzle: out of all the paintings ever made, why did THIS one become the most famous of all?
She was painted around the early 1500s by Leonardo da Vinci, who was not just a painter but a wildly curious inventor, scientist, and doodler of flying machines. Leonardo studied how light fell on skin, how muscles moved, how a real face actually looks. He was extremely good — but plenty of brilliant painters lived back then. Skill alone doesn't explain the crowds.
