The author snuck in the fun fact during a Twitter discussion about writing tips

J.K. Rowling first wrote Harry Potter and the Philosopher’s Stone on an old manual typewriter in 1995, and on Friday, she revealed where she came up with one of the manuscript’s core ideas, the Hogwarts houses.

The story of Harry Potter in the first book takes him to the school of wizardry, of course, and we learn right along with him its many quirks and processes. Chief among them is the House selection, determined by the sorting hat, in which Harry joins Gryffindor along with friends Hermoine and Ron. Each of the houses is named after the school’s founders.

Tweeting about process and typical writing habits, Rowling eventually responded to an author who said, “Years ago, at a birthday party, with no notebook in my purse I was forced to write on napkins when inspiration struck.” For Rowling, the anecdote struck a chord.

“The best thing I ever wrote on was an aeroplane sick bag,” she revealed. “Came up with the Hogwarts houses on it.”

The conversation was started by The Woman in Cabin 10 author Ruth Ware, who posted a brief Twitter thread (which Rowling endorsed) about how arguments for the best writing habits usually don’t hold up. “There’s more than one way to be a writer, just like there’s more than one way to be a human being,” she said. “Find what works for YOU, not some other writer.” She then added she didn’t carry a notebook, which spurred Rowling to chime in. (“I’ve usually got a notebook with me,” Rowling admitted.)

As to how many iterations the Hogwarts houses went through on that airplane bag, the jury’s still out. The sorting hat, among many other iconic elements, wasn’t even originally in Rowling’s initial conception of the school, and a lot can change during some flight-long scribbling. Maybe someone’s preserved the bag for all to see?