Author J.K. Rowling was your average single mom when she first got the idea for her story while stuck on a train, and even the publisher that took a chance on it couldn't have predicted that it would have a measurable effect on everything it touched.

Harry Potter propelled the young adult genre into one of the biggest forces in pop culture. It changed the business model for publishing books for kids. And it introduced an entire generation to the idea that it's possible to have a relationship with pop culture the way you want it to be — to write about it and with it, to make music and art about it, and to build a business around it. No wonder it's still going strong.

Harry Potter did fine when it first emerged in the UK two decades ago, but it started to approach phenomenon levels when Scholastic bought the US publication rights for an astonishing $105,000, about 10 times more than the average foreign rights sale at the time.

Arthur Levine, the Scholastic editor who acquired the books, had an excellent eye for British works that would be successful in the US, having already acquired the rights to Redwall and His Dark Materials. But did he have any big plans for HP? Nope. He just knew that he loved it himself and wanted to publish it. Scholastic President Barbara Marcus "kept saying 'do you love it?' and Arthur said yes, so we went for it," a Scholastic spokesperson recalled in 2002. "I would have been willing to go further than that if I had to," Levine commented in 2007.

According to Vox, the $105,000 sale granted Harry Potter two things: a built-in publicity hook, and a big budget.

The hook came from the press: Newspapers featured articles about the little English book that had garnered such a huge sale and reviewers wanted to know what kind of book would justify that kind of money.

The budget came from Scholastic itself. Whenever a publisher acquires a book, it creates a budget for it. That budget is structured so that elevating the numbers in one category means elevating the numbers in the next category: if you’re going to invest $105,000 just in acquiring a book, that means you're also going to pour extra money into marketing, publicity, and production, so that you have a reasonable chance of making that money back.

A 2012 study found that 55 percent of YA novels are bought by adults. In large part, that boom was caused by Harry Potter, which became a surprise crossover hit adored by different generations, and which made it acceptable for adults to read books that are ostensibly for children.

But for some critics, that’s a worrisome development, suggesting that adults are too stupid and/or lazy to appreciate challenging literature. Harold Bloom, a former professor at Yale and author of 'How to Read and Why', for example, though the first Harry Potter book was not well written.

"It is much better to see the movie, 'The Wizard of Oz,' than to read the book upon which it was based, but even the book possessed an authentic imaginative vision. 'Harry Potter and the Sorcerer's Stone' does not, so that one needs to look elsewhere for the book's (and its sequels') remarkable success," Bloom wrote.

But Vox's Constance Grady and Aja Romano argue that the appeal of the Harry Potter books comes from combining the intricate plotting of a mystery with the sweep and scope of epic fantasy and the intimacy and character development of a classic boarding school narrative.

They say the result is pleasurable to read at any age: the puzzle box mystery plotting keeps the pages turning propulsively forward, the fantastic mythology gives the world scope and magic and joy, and the boarding school structure makes the characters warm and familiar and charming. It also makes their eventual death (for some) and trauma (for all) deeply affecting.

One of the biggest factors that make Harry Potter so successful is its fandom. One 2011 survey suggested that a third of all American adults ages 18 to 34 at the time had read at least one of the books.

But communities like r/HarryPotter make it clear that people continue to love the series to this day. People didn’t read the Harry Potter books in isolation; they wanted to talk about it with their friends, and find more friends who loved the books as much as they did.

The books have collectively sold more than 500 million copies, making them the best-selling series of all time — with the final four novels consecutively setting records for the fastest-selling book in history.

The series has been translated into 73 languages and adapted into 8 immensely popular films, as well as spurring spin-off books and films. Something tells me we'll see many more Harry Potter memes in the future as well.