The Hook

Good afternoon, Literati!  I'm in New York for the rest of the week, so blog posts will be super short and sweet.
Today I'd like to discuss "the hook" in writing.  That special something that grabs your attention in an article or in fiction, even in a movie.  Are there certain elements of a hook that will immediately draw you in?  I'm a sucker for mysteries, throw in a ghost and some historical fiction and I'm in! 
What do you look for in a hook?  What HAS to be in that first sentence/page/scene to grab your attention?

I look forward to your responses!

Happy reading and writing!