If a five or six year old asks why the moon is round or why grass is green the usual adult answer, at least in my experience is to discourage the child. say 'what shape did you expect the moon to be, square?' or 'what colour did you expect the grass to be, blue?' instead of saying that 'those are interesting questions, lets try to find out the answer' or maybe nobody knows the answer and when you grow up you'll be able to discover the answer.