Science fiction or fantasy? A rigorous definition

If there’s a zeppelin, it’s alternate history. If there’s a rocketship, it’s science fiction. If there are swords and/or horses, it’s fantasy. A book with swords and horses in it can be turned into science fiction by adding a rocketship to the mix. If a book has a rocketship in it, the only thing that can turn it back into fantasy is the Holy Grail.

—Debra Doyle