My DD never really went through a "why" phase... she just quietly studied and observed and figured things out. My DS was the persistent questioner, and still continues to grill me to this day (he's 9). He's the mouth piece for both of them I think. His questioning has always been obscure and detailed in addition to being persistent, and very sort of sciencey and physics oriented... and often times ones I couldn't answer.

Anyway... yeah, DS is my questioner, but not DD. They're all different I think.