Every once in a while, I find myself teaching a college-level English class, and every semester that contains an October (and a few that don't), I ask my students at some point, "What's your favorite horror movie?"
The answers vary, and that's what the lesson is about: the assumptions we make when we ask certain questions. In this case the assumptions are: A) you like horror movies, B). you can pick a favorite, and, among others, but perhaps most importantly, C) we all agree on what a 'horror' movie is.
Some people will immediately head for the classics, the Canon of Horror: "Halloween," "Friday the 13th," "Nightmare on Elm Street," or even "Saw"--the movies that everyone who knows anything about horror movies know about.
Other choices are trickier. Is Alien a horror movie, or is it science fiction? Does it matter if the ghost in the haunted house story is a reptilian alien that has multiple, many-toothed maws or that the haunted house itself is a spaceship?