If you could only ask one question to each person you meet, what would that question be?

What's your favourite book and why? (Great thread btw, hope to see more replies! For the last year, we've been building an app that makes it easy to jump into conversation with new people, and created hundreds of topics based on the latest research in conversation design. My personal favourite is the book question above, as it's a pretty casual conversation starter that gets people genuinely excited.)