When I was a kid, I thought that all TV shows were broadcast live. It really confused me when I was watching Eastenders at 8pm and it was broad daylight on the show, but pitch dark outside.
I also thought that all teachers lived at school and when we went home, they would stay behind to sleep and wake up the next morning to teach us again.
And that I could dig my way from my back garden to Australia with a metal spoon. I didn't get very far.