When I was young and stupid- er, NAIVE- and a new mom, I told myself I would never answer "Because I said so." Aaaaand 11 years later.... Not gonna lie, I have resorted to it! LoL
But seriously; I always try to remember that it's a GOOD thing for a child to be inquisitive. Those are usually the brightest kids, and most creative. I think you can tell if a child is genuinely asking out of curiosity. And if they are, I would do my best to answer. Or say "I am not sure, how do you think we could find out? (try it, look it up, ask someone, etc)
On the other hand, my 5 y/o daughter quite often asks me inane questions just for attention. I know she knows the answer, and she knows I know. It is often when I am focusing on a task and she wants me to pay attention to her. In those scenarios, I remind her that I value our discussions, but that I need to get this thing done. And we can chat later.