The first sentence of Sunshine, by Robin McKinley, is: "It was a dumb thing to do but it wasn't that dumb."
Someone recommended this book to me. I hadn't read any of McKinley's books before. The situation in the first few pages grabbed my attention and the book gripped me to the end.
The sentence mentions no names, no time, no location. It does set up a problem. When someone says, "It was a dumb thing to do," we know that whatever it was, it came out badly. When they add, "But it wasn't that dumb," I hear them trying to justify their actions.
On this sentence, I wondered if many other people would share my associations. I match "dumb thing to do" to teenagers, strongly. Humans whose forebrains aren't yet fully developed are most likely to take chances, discover those chances cause problems, and then call those actions "dumb things to do." I hope that they survive, wiser, to have a second chance. It strikes close to home when they don't.
We don't yet know who is telling this tale. An omniscient narrator could be telling the story of someone else, which means the person who made the mistake doesn't have to survive to tell the tale. Even a first person narrator could end the story in a very bad place. Writers have their ways. The sentence doesn't promise that everyone or anyone survives.
Do you hear a stress on "that" when you read, "But it wasn't that dumb?" The stronger that stress is, the more it sounds like someone trying to argue that the "dumb thing" shouldn't have been such a big deal. Try reading this as a sober calculator of odds might say it, with barely any variation of stress. But "dumb thing" is not a good match to a person like that. We expect someone calmly estimating the odds to use more academic language, perhaps, "There was a possibility of an infelicitous outcome, but the risk was within acceptable bounds." "Dumb thing" is a better match to someone young and impetuous.
Someone has done something risky. They claim it wasn't that risky – but they wouldn't be calling it a "dumb thing to do" if it had gone well. We don't yet know who it was, or what they did... but we want to find out more.
This sentence draws us in with mystery and tension. It is a hook.
Graphic elements by Ken Silbert