You've probably heard it said that you need to "write what you know". This means what it sounds like: write about the things you're familiar with, that you have a good solid knowledge base on.
I disagree with this statement. Now, obviously, the people who say "write what you know" don't mean you to take it literally. How many of us have ridden a dragon, moved things with our minds, or worked on the Great Pyramid? If we literally stuck to what we knew, then the vast majority of us would be writing boring books.
If we take this un-literally, though, we're still going to end up with a world full of boring books. For example, I've never experienced anything life-threatening, or word-altering, or anything like that. I've never experienced anything that would make for an interesting book.
Another interpretation could be this "If you lived your whole life in New York City, set your book there. Don't set it in a small town in the Irish countryside, because you don't know what that's like." But I don't believe in this, either. Just because I've never lived there doesn't mean I can't capture the same sense of place, the same atmosphere.
But, then again, the phrase "write what you know" does have its value. I interpret it like this:
What you know makes for a great starting base for your writing. But it isn't a limitation; rather, it is a springboard. Expand on what you know. Expand into something bigger.
For example, say you want to write about a boy living in ancient China. You're a girl, and you obviously don't live in ancient China. You've never even been to modern China. What are you supposed to do? You're supposed to learn. Watch closely how boys interact with the world, with other boys, with girls. Do research. Try to understand their point of view. Also, research China. Research ancient China. Learn about the culture and landscape and how people lived and what they ate and what they feared and their myths and legends and on and on.
Suddenly, this has become what you know. You can write about your Chinese boy with confidence, because you have knowledge of him and his world. You set out to write something you didn't know, but ended up writing something you know.
The meaning for "write what you know" that I really take to heart, though, is this:
Every story is different. Every story has a different setting, different plots, different characters. But no matter if you're writing fantasy, or romance, or sci-fi, or "issue books", the underlying themes are the same. People feel the same no matter what kind of story you're writing. And since you're a person (I hope), you know how people feel. You know how it feels to be happy. You know what sadness is like, and anger, and loss, and hope. You know what all of that feels like. You're taking how you felt at paticular time, and passing on those feelings to your character. Therefore, you are writing what you know.
So, writers, keep this in mind whenever you hear someone tell you to "write what you know". Write what you know. Not as a limitation, but a starting point for everything else in your work. Write what you know...this encompasses more than you might imagine.