The Internet is full of visual puzzles and brain teasers, and one of the latest challenges to capture everyone’s attention is the “Eye Test,” in which participants are asked to notice the number of times the letter “m” appears in a grid full of “n”s.

At first glance, the grid seems simple enough—rows and rows of lowercase “n”s—but hidden among them are some cleverly placed “m”s that can easily go unnoticed. This test is designed to require attention to detail, visual perception, and patience.

Why are these tests so common?

Puzzles like these tap into our innate desire to solve problems and test our abilities. They’re also a fun way to break up the monotony of the day while simultaneously exercising our brains. Plus, there’s a certain satisfaction in finding the answer, especially when it seems elusive.