You can never be sure where inspiration will strike. For JK Rowling, the tale of her boy wizard Harry Potter "fell" into her head on a train to London.
But the best-selling author has revealed that was far from the strangest place for her to be seized with an idea for her books...
Rowling was on Twitter responding to a thread about writers in which author Ruth Ware observed that, unlike others in her profession, she did not carry a notebook with her everywhere.
Rowling's comments prompted another tweeter to share their experience of writing notes on a napkin – which prompted the Harry Potter author to post this gem of an anecdote:
The Hogwarts houses – Gryffindor, Ravenclaw, Hufflepuff and Slytherin – are now synonymous with Rowling's Harry Potter universe, with fans the world over identifying as one of the four.
So, it just goes to show. Wherever you may be – the toilet, your desk, flying thousands of miles in the air with a sick bag in hand – you can never be too prepared for that multi-million-selling, genre-defining, franchise-inspiring idea.