The very day that Roger asked me to give this talk, my grand-daughter stopped by my house after a science lesson. She was full of information. “Nana,” she said, “did you know that after a caterpillar has gone into its chrysalis, there’s nothing inside except soup?”

I said, “Soup?”

“Yes!” said my granddaughter with relish. “Just a lot of wet gooey stuff, and then the soup turns into a butterfly!”

Well, there’s transformation for you on a plate, as it were: caterpillar into soup, soup into butterfly. Mind you, when I checked, it wasn’t quite as magical as it sounded. The caterpillar’s body does indeed dissolve in the chrysalis, more or less, but rudimentary bits of butterfly-like legs and wings already exist inside it. So into the soup they go too, and there they grow. Nature doesn’t go in for total transformation.

All the same, this is a useful image. The reason why transformation in nature is not total is because it’s predestined, cyclical. The caterpillar hatches out of its egg on a leaf and proceeds to chew, ravenously, for about ten days, molting as it grows. At the last molt it hardens into a chrysalis, and hangs there filled with that busy soup, until three or four weeks later the shell splits open and out comes the butterfly, to flutter off and start things all over again. It’s an astonishingly complicated renewal procedure — and, of course, it’s a story.

All the prodigal cycles of nature involve this patterned transformation: seed grows to flower, which sets seed; river-water runs to sea and evaporates to cloud, to drop as rain into river; that’s how life survives, on a planet that goes ’round a sun. Our bodies obey the pattern.

But what about our minds?

What about change created by the human imagination? That’s not obeying anything. A painting, a symphony, a poem, or any fiction made of words — they’re not required for the survival of life, but my goodness, they can be powerful. Can words accomplish transformations impossible in real life? Can the mind be changed, in some totally unpredictable way? And if so, how?

Change is an integral part of story — it’s called Plot. The Horn Book issue on Transformations [May/June 2015] made us look instead from the outside, and think about changes made to stories themselves. Literary metamorphosis: books in which a modern author does new things with classic characters, from myth, folktale, or fairy story. Or from somebody else’s story — and I must say I had more pleasure from reading Greg Maguire’s Wicked than I ever did from reading the Oz stories. (But then, I’m British, and I don’t think we really get L. Frank Baum.)

The transformations include the books that give an old story a twist, so that Cinderella is gay, or her fairy godmother is a fish. Or books in which there’s a beautiful reversal, like The Three Little Wolves and the Big Bad Pig. I’ve stolen that book back from my grandchildren, for the expression on the faces of Helen Oxenbury’s little wolves when they see their nemesis reformed into a big good pig. It’s a lovely mixture of delight and extreme caution.

Or there are Jon Scieszka’s fractured fairy tales, in which the really ugly duckling grows up to be…a really ugly duck, and the princess kisses the frog and finds that he’s…still a frog. One of the reasons why Jon is so funny is that he’s a master of Anti-Transformation — he’s the Monty Python of children’s books.

Then we have the transformation of Shakespeare’s plays into stories, or into graphic novels. It’s hard to question those because Shakespeare was a great transformer himself, pinching half his plots from other people’s pages. Or there are adaptations in general, adult nonfiction rewritten for a younger reader, or a picture book made from something in another medium. This kind of transforming is done to make a story more accessible, and I wouldn’t question that either. It’s not an excuse for abridged novels, which are an abomination; you don’t cut off your child’s feet so that he can fit into a smaller bed. But a change in medium is a different matter. Think about William Joyce and The Fantastic Flying Books of Mr. Morris Lessmore. I think Joyce’s enchanting animated film is the real work of art, but his picture-book version is likely to reach many more children. You lose something, you gain something. Maybe that’s the basic truth about literary transformations.

But in the writing or reading of any book, the transformation that’s most fundamental isn’t something done to an existing story, and it doesn’t have anything to do with the mechanics of plot or character. It happens inside the imagination. The American writer James Thurber was at a cocktail party one evening — back in the days when people used the words “cocktail party” — and he was standing on the fringe of the crowd, not talking to anyone, just looking into space. He stood there for ages. And his wife came up and nudged him, and said, “Thurber! Stop writing!”

Because when you’re writing a book you are living in two worlds simultaneously; one is your own life, the other is the one you’re creating inside your head. The second is an escape from the first, but its intensity can be just as great. There’s another little writer story, about Alexandre Dumas, when he was working on The Three Musketeers. A friend came visiting one day and was appalled to find Dumas sitting at his desk in floods of tears. “Alexandre!” he cried. “What’s the matter?”

And Dumas said, “I’ve just killed Athos.”

It really is like that: when a character dies, it’s like murder. I cried at my desk once too, and it wasn’t even a fictional person I was killing off, it was a fantasy creature rather like a giant mosquito — called Peth, which is the Welsh word for “thing.”

In realistic fiction, writers seek truth by reflecting their own world in the one they create, and we respond to them because we feel it reflects the truths of our own life and experience, however changed. But people who write fantasy have chosen change from the get-go. It was wished on them, really, because they were born with the kind of imagination that loves metaphor. Fantasy is metaphor: it takes you through the subconscious to the conscious, through magic to truth.

Some of these writers jump straight into fantasy, by taking you through the back of a wardrobe, or putting you on a special train from a railroad station. Others reach the magical through the real, sometimes noticing the change themselves only when they find they’re doing it — as I did, in my twenties. I put three children in Cornwall for a nice realistic adventure story, and almost at once found them facing a character with the powers of a modern Merlin.

Like maybe half of all writers of fantasy, I have the kind of imagination that’s gone on doing that ever since: trying to put the reader into his or her own world and then changing his perception of it. There’s been just one exception in my own fifty years of books: a novel called Seaward, which showed barely a flicker of the real world before fantasy swallowed up the whole thing like a great wave. That was another kind of transformation, an example of the imagination looking for escape, because the book was written just after the worst year of my whole life, in which my marriage broke up, my father died, and six weeks later my mother died too. One of my two worlds, the one in which my children and I lived, had been so violently transformed that perhaps I had to make the other world do the same thing.

And as with writers, so with readers. We often read for escape. We read to escape from stress or anxiety or boredom; we read for the pleasure of hearing about somebody else’s problems and forgetting our own. We read to enter that second world, the one created by somebody else’s imagination, and—very important — it’s only our own imagination that can take us through the door.

This is the essential transformative quality of the book, and the reason why it will never go away, in spite of those endless predictions about the death of the novel, or the disappearance of words on paper. You can escape into the illusion of another world at the theater, or in front of a large screen or small screen; you can forget where you are, all your emotions can be engaged. But it’s not the same; it’s not the same even in the so-called “virtual reality” of video games. Words in a book can do more than even the most powerful film or play or game because they make your imagination take you into their world. Take you into it by having you make the experience for yourself, from inside. You are not hearing or seeing it, you are not relying on your senses. Reading your book, you are not lost in what your senses are giving you, you are there. In there. You’re not being made to feel; you are making the feeling.

In the theater or the cinema you may sit there at the end of a play or film emotionally exhausted, stunned. But when you come out of a book that has taken hold of you, for a moment or two you don’t know who you are, or where you are, because your imagination has been living in there, as totally involved in turning words into experience as the writer’s imagination was in reverse, when finding the right words. And going through this experience may change you—especially if you are between the ages of, say, eight and sixteen, when you are most open and vulnerable to change.

All writers, if they’re lucky, get letters or messages from new young readers saying, basically, I read your book and I loved it. (One of my favorites came from a kid in Peoria, Illinois, and it began: “Dear Susan Cooper, YOU ROCK!”) Those letters are wonderful; they give us strength, they keep us going. They also include the kind of messages that are coming to Neal Shusterman for Challenger Deep, and that surely go to writers like John Green or Judy Blume, saying thank you, your story helped me cope with life, at a crucial point when nothing else was really helping.

Over the years, writers published for children get another kind of message too, especially if they manage to live for a long time. These letters usually come from adults; they’re prompted not by that beautiful first reaction but by what Wordsworth called “emotion recollected in tranquility.” They’re looking back, and what they’re saying basically is, I read your book as a child and my world changed a little.

Generally they talk about having entered into the world of a story so completely that they absorbed its values, or learned that, contrary to all expectations, hope can triumph over adversity even when you least expect it. One remarkable girl once wrote to me at fourteen and said that a fantasy had taught her to “see the good and evil inside myself,” but these grownup reports are much less clear-cut than that. They are like people describing a remembered visit to a place which so took hold of their imagination that the feel of it stayed with them always afterwards, as they grew up.

Once in a while there are even practical changes; in my case, a few readers were nudged by the Welsh books of the Dark Is Rising quintet into taking up the Welsh harp; and two of them even became professional harpists. Others went off from America to Wales to learn Welsh, often on the edge of my home territory at the University of Aberystwyth. One of them went so far as to marry a Welsh-speaking Welshman, who had read the books as well.

But much more often than things like that, the imagination of the reader instinctively takes what it needs from a book, or books, and creates a kind of life preserver. There’s a writer named Celia Wren who had a wildly disrupted childhood, her family moving from New York to Soviet Russia when she was seven, and then to Egypt, then England, then China. She says in an essay that she survived by imagining herself part of an inseparable “band of five.”

She wrote,

I badly needed my four invisible sidekicks: blond siblings Tony and Tia; round-faced Tolly; and stocky, brown-haired Will…I’d filched them from young adult fantasy books: one character from Susan Cooper’s The Dark Is Rising series; two from Alexander Key’s novel Escape to Witch Mountain; and one from the Green Knowe novels by English author Lucy Boston…Even now, if I think about it, I can still feel what it is like to move in a band of five—a heightening of everything, as if you were living life in italics.

Her invisible friends were a bastion against fear and insecurity all through her childhood, she said. “For my sense of comfort and permanence, I would have to go back to Tony, Tia, Tolly and Will. The band of five was, in a sense, a refuge I had created for myself — one that could relocate to wherever I happened to travel.” And later on, when she was a lonely fourteen-year-old, crying herself to sleep every night at boarding school, she reached out to them again, and “as soon as I asked, they returned, as solid and reassuring as ever. It was as if I’d been on a fraying tightrope, moving through cataracts of air — and then, suddenly, I stood on solid ground.” She wrote, “Looking back on the episode 30 years later, it seems to me that Tony, Tia, Tolly and Will came through for me in a way that no three-dimensional intimate — except for my husband — has ever quite matched.”

That’s a reader’s imagination affected by the imagination of an author — three authors, in this case. It takes time for people to realize that something like this has been going on. There was once a headmaster of a celebrated school in Britain that had particularly progressive methods, and when he retired someone asked him what he felt he had accomplished. He shook his head. “We can never judge the success of any aspect of education,” he said, “until the children grow up and can tell us what really happened.”

Exactly the same thing is true for books. You discover which story really had a liberating effect on your imagination only by looking back. I bet a great many of you could look back now and remember a particular book, or books. A novel, realistic or fantasy; a picture book, with or without words. A story that was not only your escape from the world around you, but that released something inside you — and perhaps, just perhaps, made a difference during the years afterwards.

This is the ultimate transformation that we all hope for, in this world of books for the young, though we can’t do it on purpose. And maybe after all it does have something in common with those cyclical transformations that happen in the natural world, to the caterpillar, or the river, or the seed. The imagination of a child who loved to dive into stories grows up, and is able afterwards, within parent or teacher, librarian, or writer, to release the powers of another imagination. The young reading imagination, ravenously chewing up books like food.

The important thing about that caterpillar isn’t the way that he goes into his chrysalis and churns about as soup, and is changed. It’s the fact that when he emerges, in his changed state, he can fly.

From the January/February 2016 issue of The Horn Book Magazine. This article is adapted from Susan Cooper's keynote address at the Horn Book at Simmons Colloquium, “Transformations,” on October 3rd, 2015. For more on the 2015 Boston Globe-Horn Book Awards and Horn Book at Simmons, click on the tags BGHB15 and HBAS15.