What lesson, big or small, has a child taught you?
That if you ask "Why?" to every answer, you will eventually arrive at a question you can't answer.
By the way, almost everything boils down to Physics, Philosophy or Genetics. I suspect that if I were religious it would all boil down to magic (god works in mysterious ways), but since I'm not, it ends up in the top three. So I guess answering enough "Why's" tells you something about yourself as well.