I don't always do a great job, and sometimes I say 'no' (too tired, not enough time for a particularly complex answer, etc.), but I earnestly try to answer any questions my kids ask me. This means sometimes I find myself explaining topics like the nervous system to my presently-five-year-old daughter.
The question seemed simple enough: "Daddy, why does it hurt when I get a cut?" She had a small cut on one of her pinky knuckles.
When I'm feeling particularly alert and adventurous this becomes a big decision tree for me as I start to answer. Are other people in the room (particularly her little brother)? Is she in the middle of watching a cartoon? Is she eating? What's her mood? I try to provide a succinct answer first that will satisfy the basic question and, depending on her attention and reaction, I'll dig a little deeper to provide a richer picture of what's going on.
So yesterday I could have left it at "A cut on your finger hurts because your body is telling you to be careful and protect your skin" or some such. But since it was just the two of us in the car on the way home from Chipotle, I was able to tell her about nerves, and how they take messages to and from the brain (her favorite part - when your nerves are exposed to the outside world, they only message they know how to send is "AHHHHHHHH!!!!!"). I had her continued attention through explanations of the brain sending messages to your finger when you wiggle it, or your heart pumping blood. I was able to explain how you can control some of those messages and some happen without you even thinking about them. We wrapped up it up talking about how your brain is made of special nerve cells that store memories and knowledge.
I'm sure I could have done better, and I'm sure I'd make a physician cringe with some of my explanations, but after I was done and we were two minutes from home, she said, "Daddy, I want to learn science about how the body works."
Sometimes we have fights about why she can't wear a diaphanous short sleeve dress on a cold winter's day. But every now and then I get to play teacher and she gets (I hope) rewarded for her curiosity.