When we were young, our parents (when they are tired of answering) choose to answer: You will understand when you grow up / are an adult
Do not try it on your kids now. Children nowadays are too smart to fall for it. To say that it will backfire on you is an understatement 
What usually occur:
My son: Daddy, why is it like this?
Me: When the sky …. it turn to this ….
My son: But why it has to turn?
Exasperated, I answer: You will understand when you grow up.
My son (disdainfully): If you understand, tell me lar …
OR
Sometimes worst, he will say: You are old. Why you don’t understand?
Wahh… *sweats also*
Personally I don’t think there are too many things that I don’t know how to answer. Its whether those things are suitable to answer or not.
What if she ask where babies come from? Eeek!
haha~~ I also remember my mum liked to say, you’ll know when you become a mum.
But I was not a mum yet, how would I know? haha~~
Frankly speaking, I also don’t like people telling me that… I may not know when I grow up also. :p
But nowadays, kids tend to ask a lot more than we did, right? hehe~
Normally what I will answer hor is this… ” It’s like that one lar”…or if tersangkut , then refer to the daddy lor…kakakakaka…
Eve: it is like that one lar
Daughter: Huh? Like that one gar?
Ha ha! It’s true - kids are very smart nowadays!
We lived in a much simpler time back then.
he’s right wor… since u’re old edi why can’t u answer?
i tot ur son will ask ‘why’ rather than backfire like tis! smart kid!