Difference between Sci-Fi and Fantasy
I’m sure many people have their own opinions on this. This is just mine.
A Fantasy essentially has no rules. I mean, a good Fantasy story should have it’s own rules and internal consistency, but they don’t have to reflect reality or our current best understanding of how nature works.
Sci-Fi on the other hand is based on what is possible, again based on our best current understanding of how nature and the universe works (which is what science is afterall). Take AI or Alien contact for example – they haven’t happened yet, as far we know, but they could. Granted Sci-Fi tends to go for the most extreme scenarios possible with those things.
I think the lines get blurred because sci-fi often makes certain conceits in the interests of telling a story, like magic space crystals to power star ships. Or things like Star Wars being labelled as sci-fi just because it’s set in space and there are aliens. But Star Wars doesn’t explore the difficulties that may be encountered communicating with them or how interacting with alien civilisations alters the way we view ourselves – they’re just there and are goofy looking (it’s the opposite of Star Trek, which has less ‘realistic’ looking aliens, but they all have distinct needs and cultures).