[J.K. Rowling] revealed, on Twitter, another curiosity about the creation of Harry Potter and the Philosopher’s Stone. While talking about her writing habits, Rowling replied to an author’s tweet that said she was forced to write on napkins at a birthday party because she had forgotten her notebook at home.
The best thing I ever wrote on was an aeroplane sick bag. Came up with the Hogwarts houses on it. https://t.co/Fut4BvS6iM
— J.K. Rowling (@jk_rowling) December 15, 2017
Rowling wrote the first book of the series on an old typewriter, but she already said that the idea came on a train ride and that she was a costumer of several coffee shops in Edinburgh so she could develop her first drafts.