The Magic School Bus Inside the Human Body. I've read it many times since we brought it home. Surprisingly, this is one of the few things that can distract us from princesses AND Thomas.
We then went to the library, and found four more science adventures - including a journey to the waterworks.
I really would have thought the books were too "old" for them, but they seem to be exactly the right level. I highly recommend them to anyone with children who ask a lot of questions. At the back of the books, they even explain all the things that are real, and which are imaginary.
PS. There was a short lived PBS series with them as well. The film quality isn't that great, but my kids loved the clips on youtube as well.