Think you know a lot about Harry Potter?

With over 400 million books sold, the Harry Potter series was extraordinarily successful. As the books are closing in on their 20-year anniversary, we decided to dig in and find a few obscure facts for die-hard Potterheads.

1. Harry’s birthday is the real-life birthday of author J.K. Rowling.

Both author and character were born on July 31. That’s not the only piece of Rowling that became a part of one of the characters. Rowling said that Hermione is a lot like her in her younger years, so it’s no surprise that Rowling’s favorite animal, an otter, is Hermione’s Patronus. 

Really, though, who doesn’t love otters?

2. The Dementors come from Rowling’s real life battle with depression.

After Rowling’s mother passed away in 1990, Rowling struggled with depression. That inspired the idea of the soul-sucking creatures.

As she told Oprah Winfrey, “It’s so difficult to describe [depression] to someone who’s never been there, because it’s not sadness … I know sadness. Sadness is to cry and to feel. But it’s that cold absence of feeling—that really hollowed-out feeling. That’s what Dementors are.”

3. Rowling knew how Harry’s story would end early on.

While Rowling has refuted a rumor that she wrote the first chapter of Harry Potter and the Deathly Hallows long ago and kept it locked in a safe, she acknowledged that she’d developed an idea of how the final book would end long before she sat down to write the book.

In an interview with Daniel Radcliffe that included in the special features of the Harry Potter and the Deathly Hallows Part 2 DVD, she said, “I always knew—and this was from really early on—that I was working toward the point where Hagrid carried Harry, alive but supposedly dead, out of the forest, always. I knew we were always working towards a final battle at Hogwarts, I knew that Harry would walk to his death, I planned the ghosts—for want of a better word—coming back, that they would walk with him into the forest, we would all believe he was walking to his death, and he would emerge in Hagrid’s arms.”

4. Many of the story’s characters started out with different names.

You probably didn’t know that Hermione’s last name wasn’t originally Granger. While Rowling kept most of the characters’ first names, she ended up changing many of the last names pretty drastically.

Here are just a few of the characters’ original names: Madhari Patil, Mati Patil, Lily Moon, Draco Spinks, and Neville Puff. For some reason, a story with “Draco Spinks” sounds incredibly bizarre.

5. Rowling first wrote the names of the Hogwart’s Houses on an airline sick bag.

Back in 2000, Scholastic held a promotional event, during which school children across the U.S. had the chance to ask Rowling questions about the Harry Potter series. One student asked, “What made you think of the people’s names and dormitories at Hogwarts?”

“I invented the names of the Houses on an airplane sick bag! This is true,” Rowling said. “I love inventing names, and I also collect unusual names, so that I can look through my notebook and choose one that suits a new character.”