How much weight could Rapunzel's hair really hold? Could a carpet really fly? How could a modern sorceress silence a mermaid by noninvasive methods? Chris Gorski provides some solid answers over at LiveScience in a post about The Science of Fairy Tales:
The story is an entertaining read, and I'm definitely going to keep these in mind when answering questions from a budding geeklet (and future space scientist or knight, whichever comes first).