Unless it was a question worth answering my standard answer was
"When you can answer, Why the Duck Billed Platypus, than I'll tell you"
Then Platypus debate, with my insisting he will know the answer to everything when he figures it. This resulted in welcome quiet pondering.
Roughly five years later
DS "Why do we have insects"
HD: All part of the balance of life son
DS: Yes but whhhy insects?
HD: Why the DBP?
DS: Arrrrrrrrrrrrrrrrrgh!
DS: Because it is?
HD: Bloody hell son, you've finally got it!
DS: So insects are, because they are?
HD: Yup
DS: So why the Sun, is because it is?
HD: Yup
DS: Cool 
and thus ended the many years of WHY? 