Put a white piece of paper over the left side of the disk and stare at the X for 30 seconds.

After 30 seconds, take away the piece of paper but keep staring at the X.



Wow—the left side looks much yellower than the right side. The difference in perceived saturation comes from a phenomenon called adaptation. After staring at the same color for a while, the color-sensitive cone cells in your eyes stop responding as strongly as they did at first. This reduced response becomes obvious when you compare it to a "fresh" view of the same color.

Actually, there's nothing special about yellow here. It works with red, blue, green, or any other color—even puce.