There's a lot of debate about the most critical part of a book. Some writers say it's the first three pages that have to be the most compelling. Others say the first five while others would argue it's the first 16 lines. I say it's the very first sentence. The thinking is that if you can't hook a reader by that time, she will have probably closed your book and moved onto the next.
Sure, there's the title and the book jacket. (Or your query letter, if you're submitting your manuscript to agents/editors) Those are certainly hooks, too. They will tease the reader to open your book and read some more. But, in my opinion, if you don't convey enough intrigue, enough emotion in that very first sentence, especially in YA, the reader will have probably moved on. That sentence needs to reach its hands off the page and grab the reader by the collar.
When I get an idea in my head about a book, I can usually write very quickly. I become like a pitbull that can't let go until all the words are poured out onto paper. But, I'll take my sweet time with that very first sentence. I've been known to agonize over it for weeks, maybe even a month, making sure it's perfect. And sometimes it pops into my head at the weirdest times--like in the middle of the night or when I'm driving my car. Sometimes it's long; sometimes it's only a few words. It all depends. But it's got to be just right. Like the porridge.
What about you? What's the most compelling part of a book for you? As a writer and a reader?