My youngest son is amazingly curious, and he's not satisfied by simple answers. Tell him that the moon's gravity makes the tides happen and he'll want to know how it makes the tides happen because it doesn't make sense to him. Tell him that mirrors work by reflecting the light back to your eyes and he wants to know how the light knows what you look like.... (And I'm really proud that Teenage Daughter could come up with that explanation off the top of her head while putting on makeup and being pestered by her younger brother.)
How does fire work? Where do farmers get their cows? How much does a cow cost, anyway? Huh?
I can tell him where to buy a cow and about what they cost, but until he asked and I did some research I had absolutely no clue how fire works. I knew that it needed oxygen and a source of ignition, but I didn't know what was happening to that wood in the fireplace while it burned. Does it make me a horrible mom to admit that I don't really care what that flame is doing as long as it stays where it belongs?
In an effort to keep a step ahead of my kids, or at least not fall too far behind, I've been reading science books.
Why? by Joel Levy is one of the most useful that I've come across. Now that I've read it, I know where the salt in the ocean comes from (it's not the answer I was given in school and passed along to my kids), how the fire works, why the veins in your arm look blue when your blood is red, and why we get old. And a lot of other things that my seven-year-old hasn't asked about yet.
The author starts with simple explanations and then explains things further, getting to what he calls the "ultimate explanation." And he explains it all in simple enough terms that most of it didn't make my head hurt. It's written for older children, but for a mommy who's trying to explain things she never learned in school, it's a godsend.
Disclosure -- the publisher provided me with and ARC of WHY? and I'm really glad they did. Most of the review copies I leave aren't nearly this helpful!