What mystery in your own life could be a plot for a book?
Once, at the end of a conference, I walked into the bathroom of the suite I was staying in to find that the walls and mirror were covered in small splashes of blood! No pool of blood—just hundreds of small splatters, everywhere. Whose blood was it? What happened to them? Why did it happen in my room?
(I eventually found out the answers to these questions, but the mystery is far more interesting than the solution.)