New Delhi, Aug. 12 -- If you thought you had seen every kind of brain-teasing image or optical illusion, think again. A new puzzle making rounds online is being called one of the world's best optical illusions, and it has got the internet hooked. The challenge is simple on paper, but is surprisingly tricky in practice: find the face hidden among coffee beans in less than 22 seconds.

At first glance, the image is just a random pile of roasted coffee beans scattered across a flat surface. The shades of brown blend together, making it look like an ordinary coffee photo. But somewhere in that jumble, an actual human face is hiding. The illusion works so well because our brain tends to group similar shapes and colors, meaning the hidden detai...