Brain teaser, Sept. 24 -- A new brain teaser is taking the internet by storm, challenging puzzle fans and casual social media users alike to test their observation skills. The challenge is simple in concept but surprisingly tricky in practice: spot three subtle differences between two almost identical images, and you only have 10 seconds to do it.

The puzzle, posted by Captain Brain, features a cartoon of a man and two women sitting on a train bench. At first glance, the two images appear to be exact copies of each other, which is why the challenge has gone viral so quickly. Social media users are drawn to these spot-the-difference puzzles because they are not only fun, but they also test concentration, attention to detail, and quick thi...