This book answers the title question, and compares childhood to adulthood in a way that will probably ring true to children and make adults' hearts ache a little. I'm a little unsure about the illustrations, stylized in a sometimes-awkward way (a la [b:No, David!|1062516|No, David!|David Shannon|https://d2arxad8u2l0g7.cloudfront.net/books/1360058223s/1062516.jpg|562143]), but would have to test them on some kids. "The children who decide not to grow up will never grow up. They keep a mystery inside them. So that even as grown-ups they will be moved by little things: a ray of sunshine or a snowflake."