> I've always thought of it as SciFi can happen and very well might.
>
> Fantasy is a stretch of the imagination and almost positively will never happen.
>
> Drac "Am I right?" imas

I think that's the "normal" way people describe the difference between science fiction and fantasy. In principal, science fiction doesn't violate any of the known laws of science, and therefore someday, or somewhere, everything a science fiction book says happen might actually happen. Fantasy, on the other hand, utilizes "magic", which by its very nature can't actually work in real life.

Isaac Asimov's classic science fiction story Nightfall, for example, describes a race of aliens who live on a planet which has several suns, whose orbits are set up such that it's always daytime on the planet except once every thousand years (when all the suns cluster on one side of the planet). Since such a planet and race of beings might actually exist somewhere in the universe, that story might actually happen (not likely, but it is possible). On the other hand, Tolkien's Lord of the Rings involves wizards and sorcerers who can blow stuff up just by waving their fingers at them. Since there's no way in this universe at least to blow something up by wiggling your fingers and chanting some mystic stuff, that story clearly can't happen in this universe even if there is a race of little beings named "hobbits" on some planet somewhere.

Well, maybe you can blow stuff up by wiggling your fingers, especially if you're on a nuclear submarine and wiggling your fingers on a keyboard, but that's beside the point. The point is that the can happen/can't happen definition of science fiction and fantasy is convenient, and works pretty well most of the time.

But...

There are lots of science fiction stories that couldn't ever happen in this universe. For example, according to our current science, time travel might as well be magic. And, I'm no more likely to be able to travel at Warp 9 than to be able to blow stuff up by wiggling my fingers at it (again, according to current science). Since faster than light travel and time travel are used even in many "hard-core" science fiction books, talking about science fiction and fantasy as can/can't happen is clearly not a complete definition.

Indeed, what might work as a better definition is spacemen versus elves. That is, most science fiction books have spaceships, futuristic cities, aliens, fancy machines, sheet metal, rivets. Fantasy is characterized by light mists, forests, spiritual quests, swordplay, and in general, a more medeival level of technology. A book with spaceships and rivets, even if it utilizes concepts as magical as "time travel", or even "psionic levitation" (which is clearly magic, and which does exist in some science fiction books) is likely to be labelled as science fiction, while one which has horses and swordplay is likely to be labelled as fantasy.

Again, that's not perfect. Although the technology in Isaac Asimov's Nightfall isn't medieval, it isn't exactly rivets-and-spaceships, either. Still, using a combination of laser guns/swords and can happen/can't happen, most science fiction and fantasy books can be "explained away".

That's not the real definition of science fiction and fantasy, however. A science fiction book, as anyone knows, is a book that says "science fiction" on the spine, and is shelved with the rest of the science fiction books in a bookstore. A fantasy book says "fantasy" on the spine, and is shelved with the rest of the fantasy books in a bookstore. The difference between science fiction and fantasy is that the authors and publishers of science fiction call their books "science fiction" while the authors and publishers of fantasy call their books "fantasy".

In order to label these books in this way, the books must bear some resemblance to other science fiction or fantasy books. They can thus use can happen/can't happen to "explain away" the label on the spine, or lasers/swords. Sometimes, the fact that a book is written by a known science fiction or fantasy author is enough to label the book in this manner. (Terry Brooks' Running with the Demon doesn't occur in a medeival setting, and includes a guy who can "see" into a post-nuclear holocaust future of the sort that is commonly associated with science fiction. However, since Terry Brooks is a "fantasy author", Running with the Demon is labeled and shelved as fantasy.)

In general, if you're writing a book, my advice would be to write the thing first, and let the publisher decide what to shelve it as later!

[Space "Ought to cover it" Bar]