The objective of each Where’s Waldo? book is simple enough: comb through the crowds of people to find Waldo, who’s always decked out in his trademark red and white striped sweater and glasses. But simplicity isn’t on creator Martin Handford’s agenda. The English artist has made a career out of crafting immense visual puzzles, complete with crowds of people, spiraling buildings, and mythical beasts that make spotting the elusive Waldo an exercise in patience and frustration, even for the most eagle-eyed fans. Here are 15 facts you need to know about Where’s Waldo?

Waldo’s creator didn’t start his career with an eye on children’s books. One of his most noteworthy pre-Waldo works was the art for the 1981 album Magnets by The Vapors, of “Turning Japanese” fame. Despite debuting more than half a decade before Waldo, the album cover looks like it would fit right in with one of his famous look-and-find books. The album cover depicts one of Handford’s trademark crowd scenes, pulled out far enough so the swarms of people all form the shape of a giant eye.

When David Bennett, art director of Walker Books, was looking to produce a picture book similar to Philippe Dupasquier’s Busy Places series, he needed someone who could specialize in one thing: crowd scenes. While he knew Handford would be perfect for the job, someone at Walker didn’t think a book of crowds—no matter how well illustrated—would be enough. According to Walker’s character publisher, Donna Cassanova, someone at the company came up with a way to turn a crowd scene into something far more interactive for readers.

“The company was getting ready for Bologna Book Fair and, just a week or so before, someone—several laid claim to being the ‘someone’—said, ‘Wouldn’t it be good if you were looking for an individual within that crowd scene, rather than just looking at a crowd?’” Cassanova told The Independent. “Everybody thought there was something in that.” Bennett took the idea to Handford, who, in just 24 hours, created a two-page spread that the publisher displayed at the book fair. 

“In England,” Handford explained to The New York Times, “if someone says something silly or looks slightly foolish, he is called a Wally. He is a little goofy, but well-meaning.”

When the first book, Where’s Wally?, hit the UK in 1987, it became something of a phenomenon. The first four books sold more than 18 million copies worldwide in the first four years of the series’ existence, far outpacing other children’s books of the time (which would sell around 50,000 typically). Since then, the series has sold more than 55 million books, and is available in more than 35 countries and 30 languages.