There's a rule for what makes good fantasy work, and it's as...

There's a rule for what makes good fantasy work, and it's as strange as any riddle ever posed in a fairy tale: In fantasy, you can do anything; and therefore, the one thing you must not do is 'just anything.' Why? Because in a story where anything can happen and anything can be true, nothing matters. You have no reason to care what happens. It's all arbitrary, and arbitrary isn't interesting.