I was a precocious child. (Pity it's been downhill since). I used to read all kinds of books and encyclopaedias which were aimed at kids much older. When I was about seven, I read about continental shelf in a picture book.
I couldn't swim at the time. I refused to wade out too deep, because I was convinced that at the edge of the continental shelf there was a sudden drop off, and that you could accidentally step off the edge and fall in. It took years before I found out that the truth was less dramatic.