On a scaled-down football field in the heart of Chicago's Grant Park, 6-foot-3, 294-pound defensive lineman Robert Nkemdiche and two dozen other NFL superstars-in-waiting are teaching nearly 100 shrieking, gyrating boys and girls that football is a fun - and safe - game.

Pharrell's "Happy" blares over loudspeakers on the eve of the NFL draft, a three-day event that will transform 31 first-round picks into millionaires and dozens of other later-round selections into very wealthy young men.

But for these draft prospects seeking a spot on a 53-man NFL roster, putting personal safety first isn't so easy. And that's a price they're willing to pay.

"At the end of the day, the game is a dangerous game. It's a collision," Nkemdiche said after the 6- to 14-year-olds taking part in the NFL's youth football clinic finished their running, catching and tackling drills, posed for a group selfie and headed for home. "You want to do everything you can do to protect yourself and be on the line at the same time. You want to still be aggressive and still be a monster, but there's ways to do it in a safer manner. It's kind of like a paradox."

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Just over a year ago, the NFL was shaken by the retirement of one of its brightest young defensive stars, San Francisco linebacker Chris Borland, who announced in March 2015 that he was giving up football at the age of 24. He cited concerns about his long-term health following the third concussion of his career. He was followed last month by A.J. Tarpley, 23, a Stanford linebacker who played one season with the Buffalo Bills. A handful of others recently have cut NFL careers short in their prime earning years, among them Minnesota safety Husain Abdullah, Pittsburgh linebacker Jason Worilds, Green Bay nose tackle B.J. Raji, Detroit wide receiver Calvin Johnson and New England linebacker Jerod Mayo.

While these personal decisions have hardly signaled an exodus, they've added a new term to the NFL lexicon: "Preemptive retirement." On some level, football insiders say, preemptive retirement was at least a question on the minds NFL executives during the run-up to this year's draft. Which prospects, in short, were likely to bail early on their career, before the team's investment pays off? Who doesn't love football enough to play until he can't?

"I don't think it's something that really has NFL front offices up in arms," said Louis Riddick, a former director of pro personnel for the Washington Redskins who's now an ESPN analyst, when asked whether teams factor in how long a draft prospect is likely to play. "I just think they may have had to adjust their expectations a little bit."

By that, Riddick meant, considering a draft pick a success if the player fulfills a four-year contract.

Nkemdiche said he's committed for the long haul. "I'll play till I can't play no more," he said with a smile as self-assured as the all-black suit with embroidered lapels he'd wear Thursday night, when the Arizona Cardinals chose him with 29th overall pick. "You get to a point of loving the game, and you're like: 'This is me. This is what I have to do. And I know I'm going to do it for a long time."

Other top picks echoed the sentiment.

"Things happen, so you never know," said Alabama defensive tackle A'Shawn Robinson, a second-round pick of the Detroit Lions, who has played tackle football since age 4. "But I want to play as long as I can make it, honestly - maybe 10 to 16 years."

A growing body of evidence counsels against prolonged exposure to the head trauma associated with football. Concussions and repeated head trauma have been linked to chronic traumatic encephalopathy, a degenerative brain disease also known as CTE. It has been diagnosed in the brains of 88 of 92 former football players examined after death.

A study published in the Journal of Neurotrauma last month was even more troubling, finding that repeated blows to the head that fall short of concussions can be just as damaging, if not more so.

That's largely why Borland chose to retire from the NFL after just one season, despite the fact that it meant forfeiting future earnings and repaying a portion of his signing bonus. After contacting the leading neurologists in the field, he concluded that there simply were no guarantees for his long-term health as an NFL player.

Tarpley concluded the same, explaining on his Instagram feed that he was leaving a game he loved "to preserve my future health" after suffering the third and fourth concussions of his career in a single NFL season.

In recent years the NFL has taken steps to mitigate concussion risks, levying penalties for head-first hits, modifying rules on special-teams plays that pose the greatest risk and installing a medically supervised concussion protocol that dictates when concussed players may resume contact drills. Last month, after long denying a link between football and CTE, a top NFL official acknowledged that one exists during an appearance on Capitol Hill.

Many elite football players, however, profess disinterest in the latest research.

Asked if he was aware of the risk posed by concussions and repeated head trauma, Mississippi's Laremy Tunsil, a 6-5, 310-pound left tackle, said: "If you get a concussion, things happen for a reason. I don't look at stuff like that. I just try to play my game and play it to the best of my ability."

Tunsil was selected by the Miami Dolphins on Thursday after a social media firestorm touched off by a video posted on his Twitter account that showed him inhaling what seemed to be marijuana through a pipe that was attached to a gas mask. He later said his account had been hacked and that the incident occurred years ago.

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Head injury is a difficult topic for football players to broach, according to longtime agent Leigh Steinberg, whose roster of clients has included Troy Aikman, Steve Young, Warren Moon and, this year, Paxton Lynch of Memphis.

"From Little League and Pop Warner, athletes grow up in a state of denial about their physical health," Steinberg said. "They're taught early to ignore pain and that real men play under all circumstances and that being a 'training-room player' risks alienation from their peers, as well as potentially a starting position. So if young people in general are oblivious to the implications of long-term health - and athletes are that special sub-group that share the same norms - you have denial squared."

Louisiana Tech defensive tackle Vernon Butler looked bemused when asked if he'd feel comfortable telling a coach he couldn't re-enter a game if leveled by a jarring blow to the head. "I wouldn't tell the coach I'm not going back in if I got my bell rung," Butler said. "Usually I'm the one ringing the bells, so I really don't see that happening."

Butler said he intends to play as long as he can, explaining: "Your body knows when it's time to stop."

The long-term effects of head injury have long troubled Steinberg, who suffered what he called "a crisis of conscience" after Aikman sustained a brutal hit his rookie season in 1989 and was knocked unconscious in a game against Arizona.

Steinberg convened a summit about concussions in 1994. He issued an explanatory white paper on concussion awareness and prevention, calling on NFL teams to provide a neurologist on the field and to remove the head and neck from blocking and tackling. He circulated it among all 32 teams and the NFL office; it was ignored.

With an eye toward grooming the next generation of players and fans, the NFL staged a three-day outdoor festival in conjunction with this year's draft that spanned more than 20 football fields. Draft Town, as it was called, was a Willy Wonka-style world of free Ferris wheels, Skittles, Mountain Dew and Pepsi; a chance to pose with oversized helmets; collect NFL legends' autographs; test your skill in the vertical jump and 40-yard dash; and for youngsters, learn proper tackling techniques from players like Ohio State cornerback Eli Apple and Mississippi wide receiver Laquon Treadwell.

As Apple danced wildly and demonstrated proper Dab technique, Treadwell doled out pats on the back, "Good jobs!" and gentle corrections as the children hurled their tiny bodies at padded tackling dummies.

"That's part of the game!" Treadwell said, extending a hand to help an over-exuberant boy off the ground after he'd fallen chin-first, with a thud, on an ill-timed tackle.

A girl in green sweatpants and long braids beamed after doing far better.

"Tackle low! Tackle low!" Treadwell barked. "Head up! Head up!" He draped an arm around another boy and explained why it's smart to not punch the dummy head-first.

Among the eventual first-round picks who didn't travel to Chicago was Lynch, who chose instead to share the experience with family and friends at a bowling alley near his Deltona, Florida, home.

Everyone erupted when the Denver Broncos chose him with the 26th pick. It was a remarkable achievement for an athlete who never fit the NFL prototype - he was a lanky 6-7, not highly recruited, and schooled in games at a half-empty home stadium that was a far cry from NFL finishing schools like Ohio State and Alabama.

With a single phone call, Lynch joined the NFL ranks and embarked on a pro career, bearing in mind, Steinberg hoped, the conversation he had with him and his parents at their first meeting.

"We need to keep a close eye on any concussions and the amount of time before you play again,' " Steinberg said he told Lynch. And he told his parents, "Look, you love your son, and we all love football. But we need to love his long-term health more."