Aug. 6, 2009 -- Her finger hovers over the keyboard, sometimes for hours, before she painstakingly begins to type:

"You don't know what it feels like to be me, when you can't sit still because your legs feel like they are on fire or it feels like a hundred ants are crawling up your arms."

Something extraordinary happened to Carly Fleischmann, a severely autistic 14-year-old who, unable to speak, was once written off as mentally deficient.

"It is hard to be autistic because no one understands me. People look at me and assume I am dumb because I can't speak."

There are experts and skeptics who believe that nonverbal people like Carly are incapable of thinking or writing.

"I think people get a lot of their information from so-called experts but if a horse is sick, you don't ask a fish what's wrong with the horse. You go right to the horse's mouth."

Watch Carly's story on "20/20" Friday at 10 p.m. ET

Her words may never have been found if not for the relentless determination of her family, who never gave up on her. Carly's story is how one child found her way out of the dense forest that is autism, and how her experience may unlock the mysteries of this baffling disorder.

Diagnosed with Autism, Carly Lost in Own World

Born in Toronto, Carly was 2 years old when it became clear she wasn't keeping up with her twin sister, Taryn. When her parents, Arthur and Tammy Fleischmann, learned the diagnosis was autism they expected the worst -- that one of their twins would never achieve milestones most take for granted.

"When you're told your child is going to be developmentally delayed, that they might achieve the developmental level of a 6-year-old, it's like being kicked in the gut," Carly's father recalls. "And so for us, we have expectations of one child who's going to grow up and be independent and accomplish things that she wants in life, and one child, a big mystery, what's going to become of her."

In the beginning, Carly's delays prevented her from walking and sitting up, but as she grew, it became painfully clear that Carly couldn't speak. Like most autistic children, she was lost in her own world, perpetually swimming under water.

Experts told the Fleischmanns that early intervention was critical, and since Carly was 3, her therapy has been intensive and unrelenting, sometimes working with three to four different therapists daily for up to 60 hours each week. Her team, nicknamed Carly Inc., would constantly change therapies that weren't working and streamline the ones that did. Carly made small gains, but nothing that ever seemed like real progress.

"When you look in Carly's eyes, you can see an innate intelligence. So we never gave up," her father recalls.

But if there was intelligence, Carly's ceaseless rocking, flailing arms and tantrums hid any trace of it. Worse, she couldn't speak, not one word.

Intensive Therapy Attempts to Find Way for Carly to Communicate

For many years, Nicole Walton-Allen, a clinical psychologist, led Carly's therapy program.

"Her profile was that of a child who was severely autistic and more than likely moderate mentally retarded," Walton-Allen told "20/20." "I did not have any expectation that she would have a fluent form of communication. She was getting up and running around. Her hands were constantly in motion, flapping. She was drooling."

Raising a child like Carly in a house with two other children wasn't easy. Both parents were trying to make a living and keep the house going. Many nights Carly only slept four hours and, unable to control herself, still displayed disruptive behaviors. The family felt lost. Friends had suggested that they place Carly in a group home or institution, an idea her parents rejected.

"A 7-year-old in a group home, it's horrifying. The thought of that is horrifying, giving up your child. I could never do it. Never do it. How can you give up your kid? No, no. You just keep persevering, you know," an emotional father explained.

Most frustrating was Carly's inability to speak. Her therapists desperately needed to find some way for Carly to communicate her thoughts. Thousands of hours over months and years passed, and Carly's progress was excruciatingly slow.

Breakthrough: Carly Emerges from Silent, Secret World

Therapists introduced picture symbols that would allow her to communicate her needs. For example, if Carly wanted chips, she would point to the picture of chips.

But then one day, three years ago, when Carly was 11, she was working with two of her therapists when she started to feel sick. Unable to communicate what she needed, she ran to a computer and began to type for the first time.

First she typed the word "H-U-R-T" and then "H-E-L-P" and then she threw up. Her therapists were shocked: They had never specifically taught her those words, and they wondered where she had learned them.

Carly's typing showed them that there was a lot more going on inside her head than they had thought. For the first time she was able to communicate independently. After nine years of intensive therapy, and not much to show for it, Carly was finally emerging out of her silent, secret world.

When first told of Carly's breakthrough, the family didn't believe it. They had every right to be skeptical. Carly refused to repeat the exercise for her parents and her other therapists.

Because nobody apart from two people had ever seen it, the skeptics were desperate to see proof. Going forward, the plan was to use tough love to get Carly to type. If she wanted something, she had to type for it. And it worked. Several months later, Carly started to type, and what came through her finger, typing one letter at a time, with fluency that no one could believe, was simply remarkable.

Carly: "I am autistic but that is not who I am. Take time to know me, before you judge me. I am cute, funny and like to have fun."

Typing Unlocks Mystery Behind Her Behavior: 'I Want Something that Will Put Out the Fire'

Through her torrent of words, Carly began to unravel some of the mysteries behind her often wild behavior, like banging her head.

Carly: "Because if I don't it feels like my body is going to explode. It's just like when you shake a can of coke. If I could stop it I would but it is not like turning a switch off, it does not work that way. I know what is right and wrong but it's like I have a fight with my brain over it."

Carly started to realize that by communicating she had power over her environment, and she wasn't shy about expressing her desires and frustrations.

Carly: "I want to be able to go to school with normal kids but not have them getting upset or scarred if I hit a table or scream. I want to be able to read a book by myself without having to tell myself to sit still. I want something that will put out the fire."

For the first time, Carly was able to have conversations with her parents, even instant-messaging her father at his office. Her family stopped looking at her as a disabled person and instead met the funny, sassy, intelligent girl that had been trapped inside. They also said they were "horrified" that for most of her life they spoke in front of her as if she wasn't there.

Carly: "I want people to know that no one is telling me what to say and I don't have a hand up my butt like a puppet."

Despite Progress, Constant Care Still Needed

For all her progress communicating, Carly still needs constant supervision. A family member or aide is always at her side, directing her through simple daily tasks like brushing her teeth, fixing her hair, even eating. Nothing is easy. But like most teens, Carly likes music, boys, clothing and of course going to the mall.

Carly has been very clear that she sees herself as a normal child locked in a body that she has little to no control over. So in public, everything has to be broken down and planned to control her impulses. In the past she has wandered off, even stolen goods.

CLICK HERE to watch video of Carly at the mall

Side by side with her twin sister, Taryn, it would be easy to dismiss Carly as intellectually challenged. That is, until you ask her a question. For instance, Why do autistic kids cover their ears, flap their hands, hum and rock?

Carly: "It's a way for us to drown out all sensory input that over loads us all at once. We create output to block out input."

Carly's brain, unlike most of ours, is overwhelmed by the senses of sight, sound, taste, smell and touch. She calls it audio filtering.

Carly: "Our brains are wired differently. We take in many sounds and conversations at once. I take over a thousand pictures of a person's face when I look at them. That's why we have a hard time looking at people. I have learnt how to filter through some of the mess."

To help keep Carly calm -- and keep her impulses in check -- she listens to music, swims and even does yoga, which has helped with her breathing and posture.

Carly's family and therapists emphasize that no one is physically directing her to type, as some skeptics, they claim, have suggested.

"This isn't some spontaneous event," Carly's mother, Tammy, insists. "This is entirely Carly. No one is touching Carly's fingers, no one is moving anything, no one is prompting her or telepathically insinuating what she should type."

Dr. Nicole Walton-Allen, an early skeptic, concedes: "In retrospect, it is quite clear that Carly obviously had skills that we were not aware of and she needed a vehicle to express herself."

Sense of Humor, Awareness Baffle Doctors

One of the things that makes Carly so unique is her tremendous sense of humor. For instance, this exchange with her therapist:

Barb: "How cute are you?"

Carly: "I'm so cute blind people stop and stare."

Her brother Matthew is also a favorite target.

Carly: "Matthew smells so bad skunks run and hide."

Besides her obvious spunk and tenacity, Carly is empathetic and recognizes the love and sacrifice her family has made for her, something she conveyed to her father on his birthday.

Carly: "Dear Dad, I love when you read to me. And I love that you believe in me. I know I am not the easiest kid in the world. However you are always there for me holding my hand and picking me up. I love you."

"I'll go through many sleepless nights to hear that. I'll spend every penny we have to hear that," Fleischmann says.

He also notes that Carly's breakthrough was the result of thousands of dollars spent on years of intensive therapy. It's the kind of money most families unfortunately could never afford to spend on their autistic children.

A year after we first met Carly, she is happier, calmer and more independent. She also has her own internet blog and twitters regularly, answering questions from people all over North America about her experience with autism.

And Carly continues to mesmerize people by trying her hand -- that is, her finger -- at writing a novel called "The Elephant Princess."

But experts we spoke to said Carly's abilities are extremely rare and that her case should not raise false hope.

The Fleischmanns know that Carly is not out of the woods. She will likely require considerable support for the rest of her life. But Carly knows that she now has a voice that can help other kids. She looks at herself as someone who can make a mark on the world.

Carly: "I think the only thing I can say is don't give up. Your inner voice will find its way out. Mine did."

For more information on Carly, you can visit her blog, or follow her on Twitter.

Selection from Carly's Forthcoming Novel "The Elephant Princess":

"I want you to close your eyes and imagine a girl all alone in the middle of the jungle. All she can hear are the sounds of the animals. But what she does not know is that the sounds aren't just random sounds. In fact, the animals are talking to each other. People think that a lion's roar is its way to scare you. But let me tell you from experience that a roar is not just a roar. Actually a roar can mean many things depending on the tone. I think that humankind is just oblivious to things that have been around for many years. I think humans are so silly. See us animals are much smarter because we understand what is going on around us. But that's another story for another day."