ELI5: Why Does the Mona Lisa Look Like She's Watching You?

The Mona Lisa seems to watch you because the artist painted her looking straight ahead on a completely flat surface, so the angle of her gaze never changes no matter where you stand.

To understand how this works in everyday life, think about the difference between a real person and a photograph on your wall:

• A real person (3D): If your friend is looking straight at you and you take five steps to the left, you will now see the side of their face, and they will no longer be looking at you.
• A flat picture (2D): A painting does not have depth. When you step to the side, the image cannot turn. The eyes in the painting stay locked in the exact same direction relative to the surface of the canvas.

• A smartphone selfie: Take a picture of yourself looking directly into the camera lens. Hold the phone in front of you, and then tilt it to the left or right. Your picture will still appear to stare right back at you.
• Cereal box characters: Look at a cartoon character on a cereal box that is facing forward. Walk past the kitchen table, and it will feel like the character's eyes are tracking you across the room.
• Posters: A poster of your favorite movie star looking forward will seem to follow you from one corner of your bedroom to the other.