http://www.avclub.com/articles/why...
In these genres, the fundamental realities of a world can be anything imaginable: There can be wizards, or dragons, or intergalactic spaceships, or time travel, or dragon-wizards in time-traveling intergalactic spaceships. Nothing can be assumed. Which makes it mighty easy for authors to cheat by changing the rules whenever it’s convenient to the plot:
“Oh, did I not mention that dragon-wizard time-travel spaceships are sentient and can crossbreed to produce baby spaceships? Well, they can.”
Worth a look.
