I still remember the moment when I read that fossils weren't bones. Actually, I had read that several times before, but this time the text told me HOW they weren't bones.
I had the same confusion as a child. I read all about dinosaurs, but couldn't really grasp that the bones were truly long gone. I still remember being upset that many museums had casts of the original fossils on display. I wanted to see the original bones! I didn't really understand until later that at least visually there wasn't much meaningful difference.
Mind. Blown.