rethink(ip) has a nice post on parenthood::

"I have a three-and-a-half year-old son. All the parents reading this post now immediately understand the title.

Why Daddy? Why? Why? Why?

After ever explanation…he fires it right back at you. Why? The
subject doesn’t matter — here’s a brief list of topics from yesterday:
rocks, spiders, time, book titles, colors, weather, distance, pool
chemicals, bathroom etiquette, money, and (my favorite) the rules of

Most of the time, I enjoy his cross-examinations. Sometimes I’ll
have a little fun with it and give him way too much information —"

[sounds familiar]

  “…because light exhibits properties of both a wave and a particle….”
My wife usually rolls her eyes when I do this. I just wait for the
inevitable next question — why? “It’s one of the great mysteries of
quantum mechanics, son.”

Raising our four-year-old is so much fun - I love the 'why' questions.

One tip: once in while, get out the camera and reply "why do you want to know?".