[Retric]

It's not quite that simple, something can be a derivative work even if none of the original content remains and it's purpose changes dramatically. Generally speaking fan-fiction infringes copy-write when someone trys to make money off of it. However, the same work may be free and clear if you don't try and make money from it.

Teachers can generally copy something to use in class as an example. However, they may not copy the same content every year after that. Same action, but slightly different context.