Thanks, Mandi, TheyCan, and 2010Bebes for your comments and suggestions!
TheyCan, that book was actually one of the books I was looking at but I was unsure whether it would be suitable since it said ages 9-12 on Amazon. I will definitely be getting it now!
I found it quite ironic that for a chemistry major (me) and two surgeons (my hubby and me), we were stumped by her questions. I guess we were just caught by surprise that she is asking such questions this early. We didn't know how to simplify the answers. But after reading your comments, I am starting to believe that she probably does understand more than we give her credit for.