Harlan Coben uses this question to come up with all his novels: "Anytime I'm walking down the street, anytime I see somebody, I try to figure out what their story is. The what if." Here's how it inspired one of his books, "Promise Me": "I overheard some teenagers who I adored talking about drinking and driving. And I pulled them aside and I said, promise me you won't do that. I don't care if it's 3AM. I don't care what you're on. Call me. I'll come pick you up. No questions asked. I won't tell your parents." "Now in real life, nothing else happened. End of story. But fiction writing's asking 'what if?'" "What if a teenage girl had called me on the phone? I went and I picked her up in New York City. I dropped her off at what I thought was a friend's house. The next day, she's gone, and no one at the house knows who she is."