One of the great things about writing your own stories, of course, is that you have the freedom to create your worlds, then populate them with people and cultures that follow your rules.
What? You get to make up your own rules?
Yes, indeed. But there's a catch. (Wouldn't you know.)
In real life, there are always limitations that go with freedoms. If you have the freedom to drive a car, you must obey traffic laws to ensure your safety and the safety of others. When you reach a certain age, you have the freedom to live anywhere you like, but you have to pay the rent if you want a roof over your head.
In storytelling, a similar principle applies. Take magic, for example. You can imagine any magical powers you like. But if it doesn't have limits, or carry a price, it's not very believable or interesting. One sort of price might be a tremendous physical strain on a wizard every time he works a spell; he can't just cast spell after spell, like a cowboy in the old westerns, shooting an endless number of bullets.
You have freedom to use your imagination — and the more imaginative the better! But if you want your story to come to life to the reader, you have to work to make it vivid and believable. And not just the story line, but also the world, has to make logical sense. This was the challenge Hal Clement gave himself when he set out to write Mission of Gravity. He created a world with strange and exotic conditions; then he and his characters had to cope with it, and without cheating! You must start with what you already know about your world, and then extrapolate from it (extend your thinking beyond what you know) to discover additional details.
