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.

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WHAT'S GOING ON?​

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.