(CNN) Kevin Portillo practices smiling every day at home.
He hooks an index finger into each side of his mouth and pulls gently upward. He puckers his face into a kiss, then opens wide into an O, trying to limber up his facial muscles. He practices both the Mona Lisa -- slight, closed-lip -- and a wide, toothy smile.
At least, he's supposed to do his exercises every day. Being 13, he sometimes forgets, though he understands their importance.
"I need to stretch my cheeks," he says. He exercises so much that his jaw sometimes hurts.
Kevin was born in New Jersey with a rare malignant vascular tumor, a kaposiform haemangioendothelioma, covering the left side of his face, squeezing shut his left eye and pushing his nose to the right.
Immediately after his birth, doctors whisked him away to another hospital in another state. His mother didn't see him again until he was eight days old.
The doctor told Kevin's parents that the chance of him surviving was slim.
But survive he did. However, the large tumor and the damage from its treatment prevented him from being able to do one of the most fundamental things humans do.
Smile.
Most babies are born immediately able to communicate in one way: by crying.
The second signal babies send out is a smile. Newborns can smile spontaneously, as a reflex. This is sometimes misinterpreted by new parents as a reaction to their presence, a reward for their intense concern and sleepless efforts.
That new parents sometimes optimistically interpret the first reflex smiles as meaning something more underscores the duality of smiling: there is the physical act and then the interpretation society gives to it.
On a physical level, a smile is clear enough. There are 17 pairs of muscles controlling expression in the human face, plus a singular muscle, the orbicularis oris, a ring that goes entirely around the mouth.
When the brain either reacts to a stimulus spontaneously or decides to form an expression intentionally, a message is sent out over the sixth and seventh cranial nerves. These branch across each side of the face from the eyebrows to the chin, connecting to a combination of muscles controlling the lips, nose, eyes and forehead.
The basic upward curving smile is achieved primarily by two pairs of zygomaticus muscles. These connect the corners of the mouth to the temples, tugging lips upward -- often accompanied by the levator labii superioris, raising the upper lip, and other muscles of the face.
It is when we leave the realm of physiognomy, however, that the smile becomes enigmatic. This contraction of various facial muscles resonates across the entire arc of human history, from the grinning Greek kouros sculptures of 2,500 years ago right up to emoji, those little images that pepper our online communications.
Kevin did not smile on schedule. At five weeks old he was already a week into chemotherapy with vincristine, an anticancer drug so powerful it can cause bone pain and skin rashes.
Whether stunted due to the tumor or killed by the chemo, Kevin's seventh cranial nerve withered. That nerve originates at the brainstem then branches out across the face. It is susceptible not only to tumors but also to rare conditions such as Moebius syndrome, a congenital facial paralysis caused by missing or stunted cranial nerves. You can't smile, frown, or move your eyes from side to side.
"You essentially have a mask on your face," says Roland Bienvenu, a Texan with Moebius syndrome.
Without being able to smile, others "can get the incorrect impression of you," says Bienvenu. "You can almost read their thoughts. They wonder, 'Is something wrong with him? Has he had an accident?' They question your intellectual ability, think maybe he's got some intellectual disability since he's got this blank look on his face."
The challenges stemming from lack of a smile are frequently compounded. Once Kevin was able to eat food, go to school and enjoy usual childhood pastimes -- he became passionate about soccer -- he still felt the tremors of having a half-smile.
"I couldn't smile on my left, I only smiled on my right," says Kevin. "My smile was weird... people kept asking what happened to me, why I'm like this. I keep telling them I was like this when I was born."
While losing a smile is a serious blow at any age, it can have a particular impact on younger people, who are forming the bonds that will carry them through the rest of their lives.
Or trying to.
"It's a huge problem," says Tami Konieczny, supervisor of occupational therapy at the Children's Hospital of Philadelphia (CHoP). "If someone can't read your facial expressions, then it's difficult to be socially accepted. It's hugely devastating for kids. I had kids photoshopping their pictures. They are taking mirror images of their good side and copying it, photoshopping their own pictures before posting them to social media."
Photoshopping might work on Facebook. But fixing a smile bisected by nerve damage and subsequent muscle loss is far more complicated. Sometimes, it requires multi-stage plastic surgery spread out over a year or more.
There are two major procedures available for facial reanimation, according to Phuong Nguyen, a plastic and reconstructive surgeon at CHoP.
"Kevin had the most commonly done one, a classic two-stage cross-face nerve graft followed by a free gracilis muscle transfer," says Nguyen.
"My preference is to do it when kids are five years of age," says facial reanimation pioneer Ronald Zuker, a Canadian plastic and reconstructive surgeon. "At the time, if I can restore a smile for them, they can go to primary school, meet kids on the playground and in class. They have their smiles and are well-equipped to handle that situation."
Still, some parents prefer to wait until their children can participate in the decision.
"If families want to wait, that's perfectly fine," says Zuker. "Sometimes when a kid is nine or ten they look in the mirror and say, 'You know, I really want this surgery.' That's the time to do it."
Which is what happened with Kevin. He was doing well, "even with that scar on his face, has always been popular at school," says his mother, Silvia Portillo, in Spanish, speaking through a translator.
But there were kids that made fun of him, she says. One day when he was about nine, he was sad. "He said, 'Some kids, they're not my friends. They laugh at me because I look funny.' It was really hard for us as parents."
At age ten, Kevin told his parents that he wanted to do what most people do without giving it a second thought. He knew it would be a long, painful, difficult procedure, but it was one he wanted to undergo.
In October 2015, Nguyen cranked up some rock music in the operating room at CHoP.
He began work by removing a section of sural nerve from Kevin's right ankle and attaching it to the right side of his face, running it underneath his upper lip, to the paralysed left.
"We bring it across, parking it, waiting for nerves to regrow from the right side to the left," Nguyen says.
That growth took almost a year. The nerve fibers advanced about a millimeter a day (about 24,000 times slower than a snail).
During that time, doctors would periodically tap areas of Kevin's cheek, to see if the nerve was taking. "When it tingles, you know the nerve is growing," says Nguyen.
Once Nguyen was certain the nerve was functioning, it was time for the second stage of the surgery.
One morning in August 2016, he took a marker and wrote a "P" on Kevin's left temple and an "NP" on his right, for "paralyzed" and "not paralyzed", a common surgical precaution against the risk of cutting into the wrong side of a patient.
"You want to make it idiot-proof" says Nguyen.
He also drew a pair of parallel lines, marking the location of a prime artery, and an arrow: the vector Kevin's smile would take.
The surgeon made an incision from the temporal hair line, in front of Kevin's left ear, then pivoting back under and behind it, extending into his neck -- the standard location for hiding plastic surgery scars. The skin on a face peels back easily. He attached three sutures to the inside corner of Kevin's mouth and gently pulled, to gauge exactly where the muscle should be attached.
The moment Nguyen put strain on the triple surgical thread was the first time Kevin Portillo ever smiled on the left side of his face.
That done, Nguyen removed a segment of gracilis muscle from the inside of Kevin's left thigh. The muscle was secured in place by a customized splint that hooked into Kevin's mouth and was sewn to the side of his head to keep the relocated muscle from pulling out before it healed.
The section was taken from Kevin's thigh because it's rich in muscles. "There are so many muscles that do the same function... you don't miss this," Nguyen says.
Well... most wouldn't miss it. Kevin, a diehard soccer fan, did. "When the surgery came I couldn't play," he says. "I didn't know it would take that long. I thought it would take a couple days and I would be back."
Just how long was he sidelined?
"It was more than two weeks," he says, sorrowfully.
"He wasn't concerned with how serious the surgery was," his mother says, laughing. "He was more concerned with not being able to play soccer."
Over the next year, Kevin began to get motion on the left side of his mouth.
"It's really kind of a magical thing," says Nguyen. "We do this procedure, a number of hours and effort, using not a small amount of resources. We don't know if it works or not.
"I saw him post-operatively within the first couple of weeks, he looked like he had this big bulgy thing in his cheek. Nothing was moving. All of a sudden, he was smiling. It was a really incredible moment."
Well, it's magical, but it's also hard work.
Kevin begins his occupational therapy session by holding a plastic fork in his mouth and showing that he can move it up and down.
"Try to purse your lips together to make it stand up," says Anne-Ashley Field, his therapist at CHoP. "We've got it pretty solid in the middle. Try to work it over to the weaker side. Nice try... and that's harder."
Kevin puts on latex gloves and pulls at the inside of his cheek.
"You're going to do your stretch on the inside," says Field. "Do you feel like it's getting looser than it was?"
"Mmm-hmmm," Kevin agrees, gloved fingers in mouth.
She takes some photos. There is a lot of photography in facial therapy, to track progress.
Physical rehabilitation is the part of the surgical process that often gets overlooked, but it can make the difference between success and failure.
"It's huge, particularly with facial palsy," says Nguyen. "You can do technically very sound surgery on two completely different patients and have two completely different outcomes based on how involved they are with their own therapy."
How does Kevin feel about being able to fully smile after a lifetime of not being able to?
"I've been getting better on how I react. I do it automatically," says Kevin. "Sometimes when somebody says a joke, it actually feels great now. Before it felt weird to not smile. Smiling with both sides of my mouth at same time, I feel I'm one of the other people who smiles right."
His mother remembers the moment she noticed.
"We were at the table, we were eating," says Silvia. "And then we said, 'Kevin, are you moving there?' He started moving. Not the way he was doing today; little movements."
"We were eating," he says. "I think she said something funny, and I just smiled."
And how does smiling affect his life?
"Before, I was actually shy," he says. "Right now, I'm less shy, more active."
"I used to have trouble expressing my emotions. Now people know if I'm smiling or laughing. When I laughed, before, I laughed weird. And right now, they know, bit by bit, that I was trying to smile, I was expressing my laughter and my smile. When I play soccer, and score a goal, I'm happy. I'm smiling, to tell everybody I scored."