Where do ideas come from?

I was talking to my brother in law down at the beach last week about ideas.  He's a fourth grade teacher and one of the things he teaches his kids is creative writing.  I always find talking to him fascinating because he so often has to get at the origin of things.  It's one thing to talk to a teenager or an adult about writing a story because they've been exposed to them so much -- they know the basics -- but he often has to start at the very beginning and I'm not sure I'd know how to do tha...