Puzzles can be a fun challenge—until they’re so mind-bending and hard you need to call in backup. Enter the r/puzzles subreddit, where people share the toughest riddles, brainteasers ...

An orgy of thin red lines, blue smudges and black and white blobs, the abstraction lived up to its billing as the “world’s hardest jigsaw puzzle.” There were colors all right, but where did ...