I give a good explanation here.
"To douse the flames of confusion, Rod Serling greatly put into one sentence what truly separates science fiction and fantasy, which is, “Science fiction makes the improbable possible; fantasy makes the impossible probable.” For those who don't quite understand, what this basically means is science fiction, while still unreal in its implications, is based typically on real concepts or concepts that could be real, and normally attempts to make them seem real. Fantasy, on the other hand, utilizes concepts that obviously aren't real, and can pretty much go any which way it wants because it is not bound by the laws of anything. The other major distinction is the use of soft and hard sciences and technology in science fiction, while fantasy uses magic and whimsy. Sometimes the genres mix, like with space operas and the aptly named science fantasy."
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