Giant Monsters. That’s the difference between Science Fiction and Fantasy.
Of course it’s not the only difference, but it gets to the meat of the matter. Fantasy can have giant monsters like Godzilla, King Kong, Smaug and other dragons, huge spiders and insects, giant turtles and elephants that can carry a world on their backs, not to mention the 20, 50, 100 foot tall humanlike creatures that we English speakers just call “giants.” These creatures can romp across the landscape or the sky, setting fire to castles and skyscrapers, stomping whole villages flat, having battles with tyrannosaurs, or stalking frightened, heroic hobbits in the dark, and the audience simply surrenders to the wonder and the terror of such an awe-inspiring creature.
In science fiction, on the other hand, you can’t have a skyscraper-sized reptile with radioactive flaming breath, without a lot of explanations. How did Godzilla get so big? What kind of reptile did he mutate from? Why don’t his bones and muscles collapse under his enormous mass? How does he generate his atomic breath? What does he eat to refuel between rampages, since he doesn’t seem to eat any of the things he attacks?
A science fiction setting would require sound answers to all of these questions and many more, and these answers must be based in our current, real-world knowledge of biology and physics. For that reason alone, you probably couldn’t write a science fiction story with a 200 foot tall radioactive reptile, unless perhaps the story is set on a different planet with very different gravity.
If you put dragons in your SF, the same problems arise in terms of physics and biology, even if your dragons are purely reptilian. How does a dragon fly with such a heavy body? How does it breathe fire at all, let alone without burning itself from the inside? If your dragon is both sentient and magical, like the dragons of Middle Earth and Chinese myth, then you’ll need to explain not only its physical abilities, but its intelligence (not such a great leap, given what we are learning about animal intelligence) and especially its magic, in purely scientific terms.
I’m not saying, of course, that fantasy writers don’t need to explain anything! Dragons, especially, are so widely used that many modern authors have gone to a lot of trouble to create distinctive dragon biology and even dragon cultures. Anne McCaffrey’s Pern series is probably the most famous of these. My personal favorites, at least when it comes to explaining the physics of dragon flight, are Peter Dickinson‘s classic natural history, The Flight of Dragons, which I read when it first was published (though my original copy is long lost, alas); and S. Andrew Swann‘s The Dragons of the Cuyahoga, which provides a great forensic explanation of how to murder a flying dragon (and which I reviewed here in 2009).
Yet for me, half the point of fantasy is leaving some things unexplained. After all, if you understand how everything works, then what’s left for mystery and wonder?
As a worldbuilder, one of the advantages of fantasy is that you can choose which aspects of the story to leave unexplained, attributed to magic, myth or the unknown. In writing science fiction, you don’t have to explain everything on the page, but any speculative, science fictional aspects of your story do have to be plausibly explainable.
Each approach has its pleasures, of course! Which one is yours?