In most people, speech and language live in the brain's left hemisphere. Mora Leeb is not most people.

When she was 9 months old, surgeons removed the left side of her brain. Yet at 15, Mora plays soccer, tells jokes, gets her nails done, and, in many ways, lives the life of a typical teenager.

"I can be described as a glass-half-full girl," she says, pronouncing each word carefully and without inflection. Her slow, cadence-free speech is one sign of a brain that has had to reorganize its language circuits.

Yet to a remarkable degree, Mora's right hemisphere has taken on jobs usually done on the left side. It's an extreme version of brain plasticity, the process that allows a brain to modify its connections to adapt to new circumstances.

Brain plasticity is thought to underlie learning, memory, and early childhood development. It's also how the brain revises its circuitry to help recover from a brain injury — or, in Mora's case, the loss of an entire hemisphere.

Scientists hope that by understanding the brains of people like Mora, they can find ways to help others recover from a stroke or traumatic brain injury. They also hope to gain a better understanding of why very young brains are so plastic.

An injury before birth

Sometime in the third trimester of Ann Leeb's pregnancy, the child she was carrying had a massive stroke on the left side of her brain. No one knew it at the time.

Mora was born in September of 2007. And for the first few months, she seemed like a typical baby. She smiled and rolled over, right on schedule.

"And then in the holiday season of 2007, all of these milestones sort of stopped," Leeb recalls.

In early 2008, Mora began having epileptic seizures that became more and more frequent. "There were 20 of them in a minute and then there were hundreds of them a day," Leeb says.

Doctors ordered an MRI of Mora's brain. They showed the image to Ann and her husband, Seth.

"Seth and I have no background in medicine," Leeb says. "But you just didn't need it to read that MRI. Half of her brain was lit up and the other half of her brain was basically gray."

Most of the cells in her left hemisphere had died. The ones that remained were causing her unrelenting seizures. So the Leebs traveled from their home in South Orange, New Jersey, to the Cleveland Clinic, where Dr. William Bingaman performed a hemispherectomy, which removes most of the tissue on one side of the brain.

"Basically the surgery created a newborn," Leeb says. "She could no longer roll over. She could no longer smile. It was almost like a restart."

The organization of a typical human brain is contralateral, meaning the left side of the brain is connected to the motor and sensory nerves controlling the right side of the body. So Mora was initially paralyzed on her right side.

Human brains are also lateralized, which allows each hemisphere to specialize in processing certain types of information, or specific behaviors. Mora had lost the left-brain areas that usually play a critical role in producing and understanding speech. That meant her right brain would have to take on these jobs if she was ever going to carry on a conversation or read a book.

The upper bounds of brain plasticity

Ann and Seth Leeb knew that young children who undergo hemispherectomy can often thrive with just half a brain. So they devoted themselves to providing their daughter with the best possible rehabilitation services available.

Mora saw a physiatrist and a speech language pathologist. She got physical therapy, occupational therapy, and lots of encouragement from her parents.

And gradually, Mora began to improve.

"At 18 months, she finally sat up," Leeb says. "And at 23 months she finally walked."

Mora was 6 and a half when she began using sentences. By the time she had her bat mitzvah, she was able to give a short speech about living with half a brain.

People like Mora represent the upper bounds of human brain plasticity because their brains were radically altered very early in life — a period when the wiring is still a work in progress.

For example, in an adult brain, words are generally processed on the left side, while faces are processed on the right.

But "your brain doesn't start out having word recognition completely on the left and face recognition completely on the right," says Michael Granovetter, a researcher at the University of Pittsburgh.

Early on, these two critical functions appear to compete for space, he says. To give each enough room, the brain usually pushes words to the left and faces to the right.

Knowing that left Granovetter and other researchers pondering a question:

"If this competition between word recognition and face recognition in the brain plays out over development, what if only one hemisphere was available, what might we see?" Granovetter says. "Can one hemisphere take on the burden of two?"

The existence of people like Mora suggested that it could. But scientists hadn't done much to investigate the phenomenon.

A study of half-brain abilities

So Granovetter and a team of scientists set out to study face and word recognition in 40 people, ages 6 to 38, who'd lost half their brain early in life. Mora was one of them.

Ann and Seth Leeb heard about the study through the Pediatric Epilepsy Surgery Alliance, a group formed by a couple whose son had a hemispherectomy. The Leebs encouraged their daughter to participate, in part to help other people with brain injuries, but also to learn more about Mora's brain.

"We were very excited, actually, to see, perhaps, where in the brain things have migrated," Leeb says.

The researchers knew that when adults experience an injury to one side of the brain, it often results in permanent impairment. A stroke on the right side tends to impair facial recognition, while a stroke on the left side tends to affect a person's speech and language.

So the team expected to find huge deficits in people who'd lost an entire hemisphere.

"Much to our surprise, we found that that's absolutely not true," says Marlene Behrmann of Carnegie Mellon University. "Irrespective of whether the left or the right hemisphere is preserved, these kids can recognize both faces and words."

But not as well as similar aged people with two intact hemispheres. The people in the study identified both words and faces with about 80% accuracy. A control group averaged 90% or more.

The result suggests that the remaining hemispheres rewired to preserve both functions, rather than favoring the function usually found on that side. It also shows that half a brain can't fully replicate the capabilities of a full one.

Mora, for example, has some limitations, says Dr. Lisa Shulman, a neurodevelopmental pediatrician at the Children's Hospital at Montefiore in New York.

"She speaks and processes [words] very slowly," Shulman says, "And she has an almost telegraphic quality to her speech: one - word - at - a - time."

That's common among people of all ages who have an injury to the left side of the brain, Shulman says. "When you lose that left side, which is controlling a lot of motor functioning, it can impact the mouth, the tongue, the palate — how all those things come into play."

Mora also remains weak on the right side of her body, and sometimes struggles to detect nuance in language.

A mix of strengths and weaknesses

During an interview with Mora, both her abilities and deficits were apparent. So was her outgoing personality and curiosity about the world.

Mora began by telling me a joke: "How do you make a hot dog stand?" she asks. "You take away its chair."

It's a simple punchline, delivered with a pun a second-grader would probably get. But using puns at all requires a fairly sophisticated understanding of language.

Later, when I ask Mora about her bat mitzvah speech, she talks about describing her approach to life through idioms like "glass half full" and "rose-colored glasses."

Both examples show how Mora, at 15, is continuing to pass developmental and linguistic milestones, despite a slow start.

But during the interview, Mora sometimes needed prompts from her mother to understand questions. And her speech, at times, was halting and indistinct.

Her last question, though, was loud and clear: After I explain that her story will be on the radio, she asks, "When?"

The future of a half-full girl

Mora Leeb, who didn't use sentences until she was 6 and a half, now loves to watch game shows involving words and phrases.

"Do not call our house between seven and eight in the evening because we are devoted Jeopardy and Wheel of Fortune fans," Ann Leeb says.

Mora also has exceeded the hopes of the doctors and therapists who have worked with her over the years.

"Every time I see her, she's done something I could not have imagined when I first met her," Shulman says.

One of those things, Shulman says, is to develop a full-fledged personality — a girl who likes to play tennis, to have her hair put in French braids, and who occasionally talks back to her mom.

"I have challenges of being the mother of a teenager," Leeb says. "In the morning, she doesn't want to get out of bed. In the evening, she doesn't want to go to bed."

What scientists still want to know is precisely what allowed Mora's brain to rewire so extensively. Was it her age? The years of intensive therapy? An undiscovered biological factor?

One thing is clear: Understanding the basis of this sort of extreme plasticity, they say, could help millions of people whose brains are still trying to recover from a stroke, tumor, or traumatic injury. And Mora is helping scientists deepen their understanding, simply by being herself.