"The fun, and the material for this article, lies in treating the whole thing as a game. I've been playing the game since I was a child, so the rules must be quite simple. They are: for the reader of a science-fiction story, they consist of finding as many as possible of the author's statements or implications which conflict with the fact as science currently understands them. For the author, the rule is to make as few such slips as he possibly can."
—Hal Clement, Whirligig World
Speculative Fiction fanatics are always raving about how "hard" the science is in various stories -- but it's not like you can rub a story with a piece of quartz and see if it leaves a scratch on the plot. So what is "hardness" in SF? Why do people want it? And how do we put a number to it?