New Delhi, July 30 -- This optical illusion challenge has been messing with people's heads online. You have got six seconds to spot one number-4567-hiding among a screen full of nearly identical 4576s. It may seem simple, but it is not. The digits look almost the same, and your brain does not always catch the tiny switch right away.

At first glance, everything looks like a wall of numbers. Row after row of 4576, with only one 4567 hiding somewhere in the mix. It is the third digit that flips-'6' instead of '7'. And that is the trap. Your eyes skip over it if you are not laser-focused.

Most people stare for a second or two before their vision blurs. Six seconds fly by fast. Some spot it immediately. Others don't see it even after a full ...