Write what you know, your teacher tells you. Write what you know, the writing books tell you.
But say you want to write about alien mermaids in a water-filled space ship in orbit around a gas giant. Can you conceivably write about something like that?
Heck yes you can. See, you know certain things with your left-brain knowledge catalog. More than you know you know. (More on that in a second.) And you have this thing called an ‘imagination’ to fill in the rest. Write what you imagine. Research stuff you need to know about, like water pressure verses space vacuum, or gravity wells, or hyperspace.
You also know a lot more than you think you know. Nathanial Scott over on NAF wrote about things that influence your writing that you don’t even know about.
Do you know how dirt tastes?
When was the last time you put a fistful of soil in your mouth? Probably when you were a baby, right? Yet you retained the taste deep in your memory.
How about grass? How about that awful stinging in your eyes from cutting an onion? What about the shock of looking down and seeing blood everywhere and feeling no pain from an injury?
Take those things and expand on them. Your hero has a mortal wound. Do you know what it’s like to be mortally wounded? No, but you’ve been minorly wounded. Use that knowledge.
Have you ever hated anyone? Do you know what infatuation feels like? Or black rage or blind panic? You have the tools to write anything you can imagine.
Write what you know. You know more than you know you know.