Here's her secret: Your first stare at the legendary canvas will most likely be directly at the sitter's eyes. At this point, the part of your eye called the fovea that picks up fine details such as color will process the image of the eyes, while your imprecise peripheral vision will pick up the image of the lips. Because peripheral vision can't distinguish fine details, it mistakes the shadows from the sitter's cheekbones as a smile. When you return your gaze to the lips, your fovea sees the fine details of the lips. Voila! A smile turned upside down.