LOS ANGELES – Like all Southern California quarterback prodigies, the story of Josh Rosen begins with a tale about a golden arm.

It was the spring of 2013 when Steve Calhoun, a private coach, brought a half-dozen college wide receivers to the practice field at St. John Bosco High in Bellflower to work out before the NFL draft combine. Needing a quarterback to throw to the group, Calhoun stumbled upon Rosen, then a pencil-thin 16-year-old sophomore.

To hear Calhoun recount it, the ball not only jumped out of his right arm and tore through the sky that day, but Rosen’s timing seemed natural, if not effortless.

“To adjust from high school receivers to NFL (level) receivers, to throw to the different depths, that was really impressive,” Calhoun said.

As they left the field, Kenny Stills, who had been an All-America wideout at Oklahoma, approached Calhoun, struck by what he had seen.

“I wish I had him in college,” Stills told him.

The stories were already spreading. Over the next two years, Rosen emerged as the biggest name in high school football. He led St. John Bosco to a state title the next fall, lost just two games as a high school starter and became the top-ranked quarterback prospect in the country.

Rosen enrolled at UCLA early, stepping foot on campus in January as the most tantalizing football recruit in school history.

His Bruins teammates have christened him “J-Chosen.” Wide-eyed children approach him after practices with footballs and posters to sign. Las Vegas sportsbooks list him as a Heisman Trophy favorite. All of this, of course, comes before Rosen, who wasn’t officially named the Bruins’ starting quarterback until Wednesday, has taken a snap in a college game.

Physical tools, though, are only a prerequisite. Otherwise, no talented, preordained high school quarterback would ever wash out in college. It’s never that simple.

“To get the most out of Josh Rosen, you have to use his brain,” said Chad Johnson, his offensive coordinator at Bosco.

It’s a delicate balance.

Rosen’s mind stands as his greatest asset, propelling him to this stage and enabling him to process information at a rapid rate. He learned UCLA’s playbook in three days, said senior receiver Jordan Payton, who roomed with Rosen during training camp in San Bernardino. It’s why he is set to be the first true freshman quarterback in program history to start a season opener.

Yet his mind has also been conditioned to solve every puzzle and ace every test with little trouble or worry. He leans on it. It’s why he made no secret in high school that he was coming to Westwood to follow the record-setting Brett Hundley.

He flirts around the fine line between being a wunderkind quarterback and just another brash teenager.

Combative and probing

The way Rosen learned to look at the world was by questioning what anyone told him.

He tried this when watching football film. During lunch break at Bosco, Rosen and the rest of the quarterbacks often gathered with their offensive coordinator, Johnson, inside the coaches’ offices to study tape and implement the weekly game plan.

Rather than solely lecturing, Johnson drew offensive and defensive formations on a whiteboard before asking Rosen and the others to share their opinion about the play call, similar to an essay prompt.

“You have to have an open forum,” said Johnson, coincidentally a college teammate of UCLA strength and conditioning coach Sal Alosi at Hofstra. “Here’s this concept on the board. Let’s go to the boondocks. Add your two cents. What do you like? What don’t you like?”

It meshed with Rosen’s tendency to challenge other people’s ideas, when he can be a mix of combative and probing.

“That is his style,” his father, Charles, said. “That fits with his personality, to be involved, to be a part of a decision and discussion, to be in the middle of things.”

This showed up in the classroom, too. For a psychology course, Rosen wrote a paper aiming to debunk the idea of happiness. His thesis read, “Is happiness real, or are you just more content than you were the day before?”

“I think I’m a pretty smart kid,” said Matt Katnik, the class valedictorian and a friend and former teammate of Rosen’s. “But he’s a tough kid. He has a really good brain. The way he thinks, it’s pretty unique.”

It’s also ruffled feathers.

TOO CONFIDENT?

Like many bright, confident teens, there’s a tendency to try to prove one’s intelligence against others, to flaunt it. Rosen knows this well.

When he attended the Nike Elite 11 camp in Beaverton, Ore., the summer before his senior season, he sparred with Trent Dilfer, the former NFL quarterback who runs the camp. Dilfer went on ESPN, which televises the event, and issued tough comments about the young signal-caller.

“He’s super, super smart, but I think it is almost a curse for him,” Dilfer said. “Josh is a guy who has yet to buy into what I am preaching.”

The comments were, in part, driven by the fact that Rosen made corrections to a playbook the quarterbacks were given to begin the camp. Rosen believed playcalls should be made based on defensive fronts. Dilfer disagreed. And the highly publicized incident seemed to paint Rosen not as a free thinker, but as too headstrong.

In an interview a year later, Dilfer acknowledged their disagreement and continued the critique.

“I like being challenged,” he said. “I don’t mind that stuff. My bigger thing was he thinks he knows more than he knows.”

There’s little question that Rosen is self-assured.

Multiple accounts say it was that demeanor that cost him a scholarship offer to Stanford, his dream school, when he visited Palo Alto for a camp before his junior season two years ago. He was perceived to be overbearing.

“I’m too confident for my own good at times,” Rosen said in Bruce Feldman’s 2014 book “The QB: The Making of Modern Quarterbacks.” “Sometimes I do come off as arrogant in interviews or whatever, but I feel like that’s also part of what makes my play what it is.”

The Stanford camp set off red flags in recruiting circles at the time.

“I’m like, ‘Is this kid coachable?’” said Greg Biggins, an Orange County-based national recruiting analyst for Scout.com. “Is he going to be a guy people want to rally around? Or is he going to be a guy they want to duct tape to the goal post the third day of practice?”

Yet this isn’t unique, either.

“He does have an ego,” said Patrick McMorrow, Rosen’s high school math teacher for 21/2 years. “I’m not going to lie about that, but it’s a pretty strong healthy ego.”

Twenty-five years earlier, McMorrow had the experience of teaching another clearly confident athlete: former major league baseball shortstop Nomar Garciaparra.

His observation: They were both so sure of themselves. They both knew what they were chasing.

“There’s a difference between confidence and arrogance, and I like to think I can dance that line pretty well,” Rosen said. “Because if you’re not a confident guy, you’re not going to bode well on the field.”

PRODUCTIVE CHILDHOOD

It comes from his upbringing.

Josh Rosen is the middle child of Ivy League-educated parents. Charles Rosen went to Penn and eventually became a noted spine surgeon, once on President Obama’s short list to become the U.S. surgeon general. His mother, Liz Lippincott, went to Princeton and is a former magazine editor. They pushed Josh and his sisters down a similar path.

“We encouraged all of our kids to try everything, whether it was going places, trying different sports, reading about different things,” Charles said. “We always tried to make them independent and think about things. Experiment. See what’s out there in the world.”

Growing up, Josh vacationed on the East Coast with his family, visiting art museums and historical sites in Philadelphia. He met older cousins, who matriculated to Penn and Yale.

He picked up hobbies and a litany of curiosities. He learned at an early age to play chess with his father and bridge with his mother. He demanded new challenges, puzzles to solve.

As a teenager, finance piqued his interest. He talks about working on Wall Street or starting his own business after football. As a high school senior, he opened his own brokerage account. For his first investment, he poured his Bar Mitzvah money into a couple stocks, with his mom’s guidance.

He’s drawn to science and psychology. Last year, he and his dad binge-watched the science documentary series “Cosmos: A Spacetime Odyssey” hosted by astrophysicist Neil deGrasse Tyson. His favorite class in high school was AP physics. He contemplated majoring in astrophysics or engineering at UCLA.

“He was sort of schizo. ‘What am I going to study in college? I love this. I love that,’” his mom, Liz, said.

He settled on economics, with a minor in cognitive science.

“This is a kid with a global perspective,” said Yogi Roth, a former USC quarterbacks coach who was among Rosen’s instructors at the Elite 11 camp. “He doesn’t look at things from the norm. He can see himself starting as a true freshman. He can see himself leading the team.”

His vantage point is different.

LOVING THE CHALLENGE

In some ways, Josh Rosen is self-assured because his talent came naturally. It was genetics.

His father was a national gold medalist in ice dancing, competing until he was 20. He twice appeared on ABC’s “Wide World of Sports.” His mother played field hockey and lacrosse at Princeton. Rosen’s older sister, Beatrice, is a tennis All-American at Emory in Atlanta.

He started off on the tennis circuit as well.

By 2, he was swinging a racket with the help of his dad. “Just to start getting the mental circuitry of hand-eye coordination,” Charles said.

By 6, he was competing nationally and taking private lessons. By 12, he had developed nicely on the court, with a devastating serve. He became a nationally ranked player and climbed as high as No. 2 in Southern California in the 12-and-under category, his dad said.

But football enticed Josh. He stopped playing tennis in junior high – in part due to a shoulder injury – to play Pop Warner with a local team in Torrance.

He knew he had the arm to play quarterback, honed on the tennis circuit, but he grew intrigued by the X’s and O’s of football.

“He likes the chess part of football, or figuring out very quickly what the other side is doing and where he should move his chess pieces,” Charles said. “That part is what he thinks is really the bigger difference in quarterbacks.”

He keeps searching for that advantage. His mind never rests. It keeps pushing.

For Thursday walk-throughs in high school, he gathered with teammates at 6 a.m. at the Starbucks near campus to go over the game plan. He logged 7-8 hours of film on Hudl, a website that hosts game footage, Bravescoach Jason Negro said, “tripling, sometimes quadrupling, the amount of time the rest of the team was putting in.” He hung a whiteboard next to his bed in his Manhattan Beach home to draw up plays in the late-night hours. He used flash cards to test himself before games. In these instances, he wrote the name of a defensive formation on one side of the note card, then a number of plays on the other side.

“I wouldn’t say he was a football nerd,” said Bosco running back Sean McGrew, who shared the backfield with Rosen the last two seasons. “Just like the way he did his school work, he wanted to be the smartest at everything.”

Rosen isn’t a football nerd in the traditional sense. There are no pictures or videos of him wearing a football helmet at age 5 singing the “Sons of Westwood.” He never signed up for tutoring with a private quarterback coach such as Steve Clarkson. His interest in football has been more of a gradual evolution.

“He was calculated in his decisions,” Liz said. “If anything, that shows more thought, more patience, more training, more focus and directness in choosing who he wants to be. He just comes at it intellectually rather than, ‘I’m a football player. I come from a football family. This is what I do. This is who I am.’”

Football is his latest puzzle.

MAKING IT ALL WORK

It has always been about the challenge.

When Josh Rosen was 12, he learned how to string tennis rackets through a series of YouTube tutorials. So he passed on regular household chores in favor of stringing his older sister’s tennis rackets. Eventually, it turned into a side business. Liz gave him $500 to purchase a stringing machine, and they calculated how many rackets Josh needed to string before turning a profit.

This is much like his father, Charles, who searches for dizzying puzzles to solve. He enjoys the thrill of heli-skiing, downhill skiing in remote areas of the world, often only reachable by helicopter. Skiers must bring a pickaxe in case they fall into a crevice.

“They like complicated challenges, and they like to win,” Liz said. “Mentally, as an athlete, he’s a lot like Chuck. ‘Give me that mental challenge. I want to do it bigger than the average guy.’ So that’s hard to be around sometimes. It’s like, ‘Oh, my God. Do we have to do it that way?’ But it’s admirable. It’s pretty interesting.”

Josh is always searching for something to tinker with. In high school, he often came to Johnson with ideas, from play calls to overhauling the team’s offseason strength and conditioning program.

It’s not a stretch to say Rosen faces a sizable challenge this season.

He takes over for Hundley, a historic quarterback who led UCLA to back-to-back 10-win seasons. The Bruins, ranked No. 13 in the Associated Press preseason poll return almost every starter and harbor national championship hopes. Rosen needs to click, to fit in and defy history. Only one true freshman quarterback – Oklahoma’s Jamelle Holieway in 1985, when he took over for an injured Troy Aikman in the fourth game – has led a team to a national championship.

There have been much-hyped UCLA quarterbacks before, but, with the exception of Aikman (who transferred in) and Hundley, few enjoyed significant success as Bruins. Ben Olson, who was the top-ranked ESPN prospect in 2002, transferred to UCLA from BYU in 2005 after a two-year Mormon mission. But he was saddled by a myriad of injuries, never starting more than five games in a season.

There was J.P. Losman, who, like Rosen, enrolled early to participate in spring practice as a true freshman in 1999. But he fell out of the grace of teammates and coaches, according to then-UCLA coach Bob Toledo, when he insisted on taking all of the practice reps. Losman eventually spent seven seasons in the NFL, but he left UCLA after only a few months, before ever playing a down, transferring to Tulane.

Rosen is trying to better assimilate with this veteran-laden roster.

“I just have to learn how to calm down, control my emotions, play within myself and not be a hero,” Rosen said in a brief, rare scrum with reporters after a training camp practice earlier this month.

“We’ve got a really, really good team and just let them do what they’ve been doing for the last few years.”

It sounded nearly antithetical. Rosen has grown up as the hero, the alpha male raring to go, ready to fix anything. And yet he still needs to channel that too. When UCLA coach Jim Mora lit into him during the remaining minutes of a practice during training camp, spawning a profanity-filled tirade, he at one point screamed at Rosen to “take control.” He can’t coast, either.

This is the young quarterback’s reconciliation, learning not to do it all and still doing enough to lift UCLA to greater heights.

Contact the writer: jkaufman@ocregister.com