“How do you know,” a reader asked, “when a picture book is right?” She was writing in response to my blog about my most recent failed attempt to write a Christmas picture book.

In the blog I said that after two rather intense weeks of work, I had a picture book text that I loved, but even as I finished it I knew it didn't work. This reader came back to ask, reasonably enough, how a person knows when a picture book does work.

And the truth is, if I could answer that question definitively I wouldn't be staring at two weeks of unproductive work. In fact, if I could define that mark before I got there I could hit it every time.

For many years I worked with an editor who never wanted me to tell him what I was working on except in the most general terms. He wanted to reserve for himself that instant of feeling what it all means when he got to the end of a story for the first time, the moment when, if the story is working as it should, the hair will stand up in the back of the reader's neck. And that is what we're always writing toward, that frisson that skitters across the skin when a story draws to its just-right conclusion.

In other words, your heart tells you. If a picture book works—if any story works—something about it moves you, changes you, gives you back to yourself new. That's why if a picture book is right for a child he may demand to have it read a hundred times, a thousand. He wants to feel that moment when everything clicks into place again … and then to feel it again … and again.

How do you know when you have created that magic moment? That click, if your story truly falls into place, happens inside you first. The problem, though, is that you can't always be certain that the click you experienced will reach your reader.

So first we can ask ourselves if our story transformed us. If it didn't we will always know we have missed the mark. Next we can ask trusted critics. Those people can range from other professionals—agents, editors, fellow writers, teachers, librarians—to anyone in your life whom you've discovered has an instinct for recognizing that moment when a story is exactly right. My life partner, who has never spent time with children since she herself was an adult, has a very solid instinct for it. She may sometimes like a manuscript that I find I can't sell, but she has never mistrusted one that people who mattered thought worked.

Our least reliable critics are usually children themselves. That's not because children don't know what they like, but because they are often too eager to please. They say what they think the adult consulting them wants to hear. On the other hand if you can find children in a situation where they are not intent on pleasing, their candor can be a gift.

A friend of mine was once told by an editor that her picture book manuscript was much too long. Indignant, she said to me, “What does he know? Does he even have kids? My grandson loved every word!” Not long afterward, she had an opportunity to read her manuscript to a kindergarten class. When she finished reading, one little boy in the back row yawned, stretched, and proclaimed, “That's the longest story I ever heard in my whole life!” She came home and revised her picture book.

But whatever good critics can tell you, the real secret to picture books lies in the heart. What does it make you feel? Would it make you want to live the story again? And again?

And what touches the heart cannot easily be defined or taught. It can only be discovered … each time new.