Leonardo da Vinci was not just a painter. He was a scientist who believed that understanding how things worked made his art better. When Leonardo died on May 2, 1519, he left behind paintings that changed art forever. What made Leonardo different from other artists? He studied anatomy -- the structure of the human body -- by examining real bodies. Leonardo wanted to know exactly how muscles, bones, and tendons fit together under the skin. This knowledge made his painted figures look more realistic than any before them. Leonardo also invented a painting technique called sfumato. Instead of drawing hard lines between light and dark areas, he blended colors so softly that the edges seemed to disappear. The Mona Lisa's famous mysterious smile works because of sfumato. If you look at her mouth directly, the smile seems to fade. Look at her eyes, and the smile appears again. Leonardo spent years on some paintings. He worked on the Mona Lisa for at least four years and may have kept refining it until his death. Today it hangs in the Louvre Museum in Paris, where millions of people see it each year.