Diana Renn: Sometimes mysteries brush up against us and feel uncomfortably close. Two years ago, a colleague of mine in the textbook writing business went missing. She boarded a Washington State Ferry and never got off the boat. Her car was found unoccupied on the ferry dock, her purse, wallet and keys on a table in the dining area of the ferry. They searched the cold waters of Puget Sound, but a body was never recovered. Foul play was initially suspected, but then a note was found sometime later, and signs pointed to suicide. Yet many people who met or knew this cheerful, ambitious, high-achiever -- who was also a wife and a mother of two -- and an accomplished swimmer, oddly enough -- were left with questions about the circumstances of her demise. I wasn't close to this person, but had met her several times at conferences and received professional advice from her over the years, and we'd worked together briefly on a publishing project. So when I see her name on book spines, I still feel that jolt, and that simple question zings back at me: why? I don't feel compelled to write about this mystery in fiction. But thinking of that glaring why? guides my mystery writing. Why? is like a blinking neon sign above my head when I work. It compels me to ponder motives and causes, to carefully consider the reasons behind my characters' puzzling behaviors.