The Mona Lisa is a famous example of a painting with eyes that follow you. How does she do it?

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It must have startled the first people who noticed it: The eyes in some paintings seem to follow you. We've been aware of the strange phenomenon for a long time now, and it's made its way into popular culture. All sorts of crooks have been able to keep tabs on Scooby-Doo and the gang without being noticed, simply because the young adventurers take it for granted that the eyes following them from the paintings they walk by aren't real. A "Haunted Harry Houdini Painting" you can purchase takes the illusion to its most extreme point. The Mona Lisa is as famous for her weird ability to follow you with her eyes as she is for her puzzling smile. And conceptual art group Flong has created a robotic art installation featuring an unsettling eye that really does follow you across a room -- and blinks!

We all know that some paintings seem like they watch us, but how exactly does this happen? Why does it work for some paintings, but not others? It turns out that it has to do with the way a painting is created and a canvas' lack of the third dimension we find in real life. Thanks to the elements of shadow, light and perspective, some paintings give us the uncanny feeling of being watched. It's only fair, if you think about it. We like to look at paintings, why shouldn't they get to look back at us?

Before we get to the bottom of this phenomenon, try a little experiment. Ask a friend to stand still and stare directly forward. Now move slowly around your friend, always keeping his or her eyes in view. Do they seem to follow you? No? Aha, we have a clue. So this optical illusion happens only in art, not in real life. Why? Read the next page to find out how a group of scientists finally solved a centuries-old puzzle.