At some point after the "that" phase comes the "why" phase. That is fun for awhile, but can also get annoying. I would frequently put the question back to my kids - "why do you think?" That would usually result in one of two outcomes - 1) they simply stopped asking, which was nice for my annoyed self, or even better 2) they would actually think about it for themselves, and often come up with the same answer as I would have, or maybe something different I wouldn't have, and then we could really have a conversation rather than just an FAQ.