Harry Potter author J.K. Rowling revealed on Twitter that an airplane vomit bag was the best writing tool she ever had while conceiving the series. In fact, it was on a sick bag where she first wrote the four Hogwarts houses of Gryffindor, Ravenclaw, Hufflepuff, and Slytherin.
The factoid of Potter trivia came from Rowling quoting a tweet from In a Dark writer Ruth Ware, who said that online writing tips are basically useless. For instance, Ware said that even though she's a writer, she doesn't carry a notebook everywhere she goes. Rowling quoted the tweet to say she agreed with it, except for the part about the notebook. Her main point was that all writers have their own practices and that you can't distill an author into a list of common habits because everyone is different.
The Hogwarts houses are a major part of the Harry Potter books, so it's cool to see Rowling going into their backstory. All first-year students are sorted into the houses via the Sorting Hat, which was created by the original founders of the school. Gryffindor, Potter's house, values bravery, Ravenclaw values knowledge, Hufflepuff values fair play, and Slytherin values ambition. Specifically, Voldemort was in Slytherin, the house that created more dark wizards than the other three.
The idea of going beyond the expectations of one's house is an underlying theme throughout the books. For instance, Tonks is incredibly brave for a Hufflepuff, while Peter Pettigrew is incredibly cowardly for a Gryffindor.