India, Sept. 17 -- Optical illusions have a way of pulling us in, and the latest one making rounds on social media is a perfect example. At first glance, it looks simple: a big grid filled with identical green apples. Neat rows, clean columns, all the same bright fruit. But hidden inside is an intruder - a single green pear tucked among the apples.

The grid is big enough to test anyone's patience. It stretches 21 columns across and 11 rows down, each spot filled with the same repeated apple image. The odd one is so well-blended that scrolling eyes tend to pass right over it. It's the kind of brain teaser that sounds easy but ends up chewing through more minutes than you'd expect.

Puzzles like this hit on something basic. Our brains are ...