New Delhi, Sept. 8 -- A new brain teaser, now circulating online, shows a plain grid filled edge-to-edge with the number 23. Twelve columns, seven rows, nothing unusual at first glance. It looks almost too simple. But here is the catch. Hidden somewhere in the sequence, one square does not match the rest. Instead of 23, the number reads 33. And that tiny swap is proving tougher to spot than most expected.

The trick lies in repetition. Once the brain sees the same number appear dozens of times, it stops paying close attention. Psychologists often call this "visual fatigue." Your eyes are scanning, but your mind goes into autopilot.

Plenty of people online admitted they scrolled back and forth through the grid without catching the change....