Um... clean diapers and dirty diapers?

A friend of mine, a better writer than I am, is working on an article about the joys and challenges of raising children. She asked me for a quote. Here's what I said:

The most joyful thing about raising a child is hard to define. One day it's seeing the light of discovery in her eyes when she learns something new. The next day it's hearing her laugh, or watching her dance. The next it's listening to her stream-of-consciousness babbling over supper.

And I guess that's the most challenging thing about raising a child, too: always reminding yourself that the kid knows more than you think she does, and always remembering that she'll learn far more from what you show her than from what you tell her.

I've got less than two years' experience as a parent, so I'm curious to know what the seasoned veterans out there would say. For that matter, I'm curious to know what people who don't have kids would say. What do you think is the most joyful thing about raising kids? What is the most challenging thing? Think it over and put it in a comment on this blog!