Doesn't it seem like every famous author or artist has some dramatic life history? You hear all about how this one struggled with depression and alcoholism, how that one decided to go live in the wild one day, how so-and-so was the daughter of an actress and traveled all over the world. Here's the thing: we can't all have a totally fascinating and interesting life. Some people are, in fact, basically pretty normal: suburban neighborhood, standard high school, part-time summer job working at a fast food joint. So when you hear trite advice like "write what you know," it's perfectly fine for you to roll your eyes.
Not all of us know what it's like to see the hills of Spain lit with the afternoon light (lookin' at you, Ernest Hemingway) or grew up listening to fascinating Civil War stories from people who were actually there (but good for you, Margaret Mitchell). Some authors can only dream of being on a fully-rigged sailing vessel, storming across the seas...but should they never write about it just because they've never done it?
No. The much-parroted advice write what you know doesn't necessarily mean that you should go roam around a drafty English castle before you pen that romance novel, and I definitely shouldn't run right out and kill someone before I craft my next mystery book. A great many authors have very successfully written historical fiction novels; obviously, none of them are in possession of time machines. And while I can't say I ever knew the man, for he died well before I was born, I'm pretty darned certain that Jules Verne never actually went to the center of the Earth or 20,000 leagues under the sea, either.
Live and die by the write what you know cliche, and you're limiting your range of topics severely. If everyone followed that adage to the letter, it's extremely unlikely the world would have any science fiction stories, vampire romances, or Stephen King (unless you want to believe he was actually attacked by fog while at the supermarket).
However, write what you know is still decent advice -- but only if you understand that sentiment for what it is.
What Do You Know?
Whether you're writing about a girl named Sally, talking animals who live on a farm or an inanimate object (and those are all real books, by the way), there is always some human truth in the tale. After all, love is love whether it's happening in a drafty castle in 14th century England or in a college English class in Michigan. If you've really been in love and you really feel like you know romance, write that romance novel. Set the scene in an enchanted forest, 200 years in the past, or even on the moon if you like. If you know love, you can pull off a love story -- use your research skills to fill in the blanks when it comes to setting, daily life, and what gravity is like on the moon.
Write what you know is really just an easy way of saying that all writers have to draw on their own experiences and their own histories, wrap that all up in fiction and put it on the page. If you have a very complicated love-hate relationship with your sibling, it can become interesting conflict in your next book. If you've ever had a crush on someone who didn't crush back, you can include that unrequited experience in your work -- and, because you actually had that experience, it will feel very believable. That situation is universal, and it can happen on a pirate ship in 1760 or a futuristic colony on Mars in 2760. The emotions you've felt are always real, and they can always be used in your writing -- whatever you might be writing about.