Has anyone ever said you should write a book? Maybe extraordinary things have happened to you, and they say you should write a memoir. Or you have an extremely vivid imagination, and they say you should write a novel. Maybe your kids are endlessly entertained at bedtime, and they say you should write a children’s book. Perhaps you just know how everything should be and imagine your essay collection will set the world straight.

Everyone has a book in them, right?

I hate to break it to you but everyone does not, in fact, have a book in them.

I am a literary agent. It is my full-time job to find new books and help them get published. When people talk about “having a book in them,” or when people tell others they should write a book (which is basically my nightmare), what they really mean is I bet someone, but probably not me because I already heard it, would pay money to hear this story. When people say “you should write a book,” they aren’t thinking of a physical thing, with a cover, that a human person edited, copyedited, designed, marketed, sold, shipped, and stocked on a shelf. Those well-meaning and supportive people rarely know how a story becomes printed words on a page. Here’s what they don’t know, and what most beginner writers might not realize, either.

Every story is not a book.

A story may be things that happened, embellished for interest, but that’s not a book. Many stories don’t get good until the end. Some stories — true ones even — are hard to believe. Other stories are just too short, don’t have enough tension, or frankly aren’t that interesting. The stories we tell that enrapture our friends and families may be extraordinarily boring to those who don’t know us. Those stories are not a book.

A book may also be things that happened or that we wished happened, embellished for interest, but it’s also so much more. It’s a story told artfully on the page, tailored to the reader. A book has a beginning, middle, and an end that keeps the reader invested for the five, six, ten hours it can take to read a book, because if it gets boring in the middle, most people stop reading. A book, when published by a traditional publisher to be sold in stores, has a defined market, a reader in mind, and that reader is one who usually buys books, not just some hypothetical person the publisher hopes to catch off the street.

You can tell a story to anyone who’s willing to listen. But writing a book that people will pay money for or take a trip to the library to read, requires an awareness few storytellers have. It is not performance, not a one-person show. It’s a relationship with the reader, who’s often got one foot out the door.

Writing is hard.

Remember writing papers in school? Remember trying to eke out 1,000 words or three pages or whatever seemingly arbitrary number a teacher set? Remember making the font bigger and the margins wider? You can’t do that to a book. I ‘m often sent stories that are way too long or too short for the publishing industry, and that makes them bad candidates for books. The average novel, for adults or children, is at least 50,000 words. That’s 50 three-page papers. Shorter books are not cheaper for the publisher to make, for many reasons too boring to get into here, and no, it’s not just cheaper to do ebooks, either. (No, really, it’s not.) If you’re an epic writer and think breaking up your 500,000-word fantasy series into five books is the key, you’re wrong there, too. A publisher doesn’t really want book two until they see how book number one is selling. And if your story doesn’t wrap up until book five, then you’re going to have nothing but disappointed readers. Writing — just getting the words on the page — is hard, period. Writing artfully so that someone enjoys what you’re writing is even harder.

Publishing is a retail industry, not a meritocracy.

Writing is an art form, books are art, but they exist in a system that relies on readers to exchange money for goods. That money pays a publisher’s rent and electric bill, and the salaries of the often hundreds if not thousands of people they employ to make the books readers buy. And if a book doesn’t make money, it’s very hard to pay those salaries. Publishers take a financial risk on a book, because no one knows how a book is going to sell until it’s on shelves, and very successful authors (your JK Rowlings and James Pattersons) help pay the bills for the less successful books. Publishers certainly publish books they know are not going to make a lot (or any) money, and they do this for the sake of art or history or prestige, or a dozen other reasons. But they can’t do it that often. So, you may have an amazing story, but if there isn’t sufficient evidence that readers will flock to it, you’re not likely to be published. No one deserves to be published just because they completed a book. It’s not “if you write it, they will come.”

Just because you are fluent doesn’t mean you can write.

If you’re reading this, you can very likely write. You’re probably fluent in English, or very close to, and you are able to convey ideas using words. But that doesn’t mean you can write a book.

I put it like this: I have been running since I was about a year old. Almost 40 years! But I cannot for the life of me run a marathon. I am not physically capable of it, even though I can run a few miles in a row. Writing a book is a marathon. You have to train for it, practice, understand your strengths and weaknesses and work hard to overcome them. You need help, feedback, and support, and you need to try many times before you run your best race. Writing a book that someone else wants to read is running your fastest marathon. No one does it right out of the gate, and few writers can expect to have the stamina without rigorous training.

If you want to write a book, do it. It’s wonderful and horrible and fulfilling and soul-crushing all at the same time. But do it because you want to, not because someone suggested it one time. Be mindful of what it fully entails before you start, so you have reasonable expectations and set reasonable goals. You don’t have to write with the aim to get published, and you don’t have to publish with a traditional publisher. There are many options if you just want a copy of your story that you can hold in your hands. Just be careful when well-meaning, though wholly uninformed, people say you should write a book.

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Kate McKean is a writer and literary agent in Brooklyn.