OK, so a book that starts this conversation is GREAT!!
Sitting at traffic light in car, looking at the sunset:
5 yr old: Mommy did you know, the sun is a giant ball of gas?
Me: Why, yes it is...[thus ensues a conversation on the size of the sun compared to the earth, other planets in the solar system, etc.)
When I asked her if they were talking about the sun/solar system in preschool, she said no, she was reading about it in the Magic School Bus book.
Can I just say - AWESOME!!!! = )
Doesn't hurt that they also watch the MSB shows occasionally as well - good reinforcement of the topics.