Charles Rex Arbogast/Associated Press

LAKE FOREST, Ill. — See Tarik Cohen in practice; understand the concept of kinetic energy.

He darts and dashes, a rabbit on Red Bull. During warm-ups, a backflip, just to stretch the back. His mouth is running, rapping, roasting. Give him the ball, his legs move faster than the eye. The play ends, he sprints to the end zone. On the sideline, he dances.

The Bears use a GPS system from Catapult that monitors the exertion of their players during practice. He consistently scores the highest on the team, according to coach John Fox.

"He never stops moving," Fox says. "I saw him run off the sideline today, and he actually danced left and right probably another 30 yards to get back to the huddle. He's a ball of energy. And his personality is the same way."

Cohen shrugs. "I just want to have fun," he says. "Sitting still isn't too much fun."

He's the same way on game days, when the rookie has the effect of an ammonia cap on his teammates. "His personality is infectious, and it gives us all a spark," guard Kyle Long says.

It started against the Falcons in Week 1, when he took a pitch and started left. Two defenders crashed through his potential lanes, so Cohen put on the brakes and cut right, up the right sideline and turned on his 4.42 speed. He broke a tackle before being brought down for a 46-yard gain that might have covered 75 yards. Then, it seemed, all of his teammates on the sidelines hit personal bests in vertical jumps.

It didn't stop there, and it can't be a coincidence that in each of the Bears' upset victories this season, Cohen made a play that changed the game. On the second play of overtime against the Steelers in Week 3, he had an incredible 36-yard run that would have been an even more incredible 73-yard game-winning touchdown run if officials had not made a questionable ruling that he stepped out of bounds. Against the Ravens in Week 6, he threw a touchdown pass. Then he skipped to the sideline. Against the Panthers in Week 7, he caught a 70-yard pass—the Bears' longest reception of the season by 25 yards.

Cohen is more than a running back. He's a life force.

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When Cohen showed up for the first organized workouts with veterans in the offseason, many of his teammates had the same reaction.

"Man, this is a small guy," says defensive tackle Akiem Hicks, who has about 11 inches and 151 pounds on the 5'6", 181-pound Cohen.

Then they saw him play.

"The first play I went against him in OTAs, he was running one way, and I was chasing him, and he cut back," Bears outside linebacker Sam Acho says. "I was like, I could not have possibly made the tackle on this guy."

After one of their first spring practices, fellow running back Benny Cunningham had to tell his former Rams teammate Todd Gurley about Cohen.

"This running back is sick," Cunningham told Gurley. "You have to start keeping up with him."

Not long after, Hicks says, there was a chirp around practice. Defensive players were jockeying so they wouldn't have to cover him one-on-one.

Nam Y. Huh/Associated Press

It's nothing new for Cohen. He's accustomed to converting doubters to believers. Almost everyone thought he was too short to play in college, let alone in the NFL.

Back when he was in high school, one college offered him a scholarship only to rescind it after one of the coaches saw Cohen in person. In the end, only North Carolina A&T wanted him.

He saw the skepticism on social media forums, too. So he made a fake Twitter account using his cousin's name. "What do you all think of Tarik Cohen?" he posted. "Can he play D-1? Could he make it at Alabama?" The responses were predictable.

Despite running wild in the Mid-Eastern Athletic Conference, it was well into Cohen's senior year at North Carolina A&T when he finally realized that he had a chance to be drafted. But he knew he would not appeal to every team. The Panthers offices are just a pleasant drive from Cohen's college campus, but they never seriously considered him, according to their head coach Ron Rivera.

Cohen was the smallest prospect at the combine. Now he is the smallest player in the NFL. And he probably is the smallest man when he walks into a Wendy's, where he eats like a bigger man with his go-to order—the four for $4 special.

At family gatherings, however, Cohen stands tall. His mother, Tilwanda Newell, is 4'11". His brother Tyrell, a fraternal twin, is 5'4". His older half-brother Dashawn Clayton is 5'6". His younger half-brother Danta Norman is 5'4".

Fox has said Cohen is short but not small. His waist size is about 33 inches. He wears size 10 cleats.

Especially, his hands are not small. They measured 10 ⅛" at the combine. Of the 33 running backs there, only one—Matt Dayes—had larger hands. For comparison, 6'0", 240-pound Leonard Fournette's hands measured 9 ¼".

"These hands are good for catching the ball," Cohen says. "They also are good for smacking the fire out of people when they talk crazy to me."

Have you ever seen a Chihuahua be a boss with much larger dogs? That's Cohen—a big dog in a little dog's body. "Everybody else sees me as small, but I don't see myself that way," Cohen says. "I can't see myself, I'm just running the ball."

Against the Falcons, he put cornerback Desmond Trufant on his back as he barreled into the end zone. "He's strong," Bears running backs coach Curtis Modkins says. "If you watch, defenders bounce off him more times than not. He has a natural low center of gravity. He has natural pad level."

Physics being what it is, Cohen is not going to win every collision. His college coaches told him to get out of bounds at the ends of runs, but his Bears coaches have told him no such thing. And he likes it that way.

"I feel like if I'm playing football, collisions are a part of the game," he says. "If I'm scared of that, I shouldn't be on the field. I don't let too many players get solid hits on me, the way I turn my body—I don't give them a solid surface to hit so they can't get a complete blow on me. I haven't had a real hard hit yet at the NFL level."

He has proved durable thus far. In college, he missed only one game with a knee injury despite 972 career touches.

The only time his twin remembers him getting hurt was when Tarik was about 13 after dunking on an eight-foot basketball rim.

"He was hanging on it like Spider-Man," Tyrell says. "Then he fell straight on his head. He didn't play no more that day, but he thought he was the man."

Still does.

So far this season, Cohen has lined up at seven positions—running back, F receiver (slot), X receiver (split end), Z receiver (flanker), quarterback, punt returner and kick returner.

Cohen will play anywhere. He started out as a middle linebacker in youth football. When he got to middle school, he played safety and cornerback. And then coaches tried him at noseguard—yes, noseguard. "I was the smallest guy on the team but the quickest, so they couldn't get their hands on me," Cohen says.

Who knows where Cohen will line up next? "His biggest asset is his versatility," Modkins says. "He can do a lot of things. He can run a lot of routes backs normally can't run. He can absorb a lot of information."

Most rookies struggle with one position, let alone seven. Cohen works at it, and he's capable of absorbing quickly. "To be able to work hard and be enthusiastic with it is unique," Fox says. "He has that quality."

He also has explosive ability. It was a YouTube stunt that put him in the public consciousness. Cohen's teammates goaded him into trying to catch a football while doing a backflip. He did it, they taped it and put it on social media.

Then, Cohen said, someone else posted a similar video. Cohen had to one-up him. Thirteen takes later, Cohen had the now-famous video of catching two footballs while doing a backflip.

For his next trick, he fired a basketball nearly the length of a court. Swoosh. That video went crazy on social media, too. "I do everything," he yelled at the end of the clip while pounding his chest. "I do everything!"

Cohen also posted a video on Instagram of him dunking on a regulation hoop. "I played rec league basketball one year," he says. "I was like LeBron out there."

Modkins coached Jamaal Charles and Reggie Bush previously, and he sees some of each in Cohen. Bears general manager Ryan Pace, who bravely chose Cohen in the fourth round of the draft, is one of many who has compared him to Darren Sproles.

Against the Ravens, Cohen was like Wee Willie Smith. When Cohen threw his 21-yard TD pass, he became the shortest player to throw a scoring strike since the 5'6" Wee Willie did it in 1934. "Shoutout to Wee Willie," Cohen said afterward.

It's difficult to say what his most impressive feat has been.

"The touchdown he threw was pretty unbelievable," Modkins says. "I don't know if you can throw it any better. That run against the Steelers, there are probably only a couple of guys in the league who can do what he did. The route he ran against the Panthers, the corner post route, it normally takes years to get that right. He ran it about as good as you can run it."

Only lack of imagination stands between him and his next wow moment.

A couple of weeks ago, Cohen had his braces removed. Now, in his glory days, he can show the world his smile.

In a league that has been shackled by anger, Cohen is all about the smiles—his and ours. There is no chip on his shoulder, no dark motivation behind his big plays. "The style of play I have, I can't be an angry runner," he says. "You don't think of somebody juking people and being mad. When you are juking people, or having explosive runs, it's a happy thing."

Impacting the game in a different way every week Week Result Cohen highlight(s) 1 L 17-23 13 touches, 113 yards from scrimmage, receiving TD 2 L 7-29 8 receptions for 55 yards 3 W 23-17 12 rushes for 78 yards 4 L 14-35 10 touches, 48 yards from scrimmage 5 L 17-20 4 punt returns for 25 yards 6 W 27-24 21-yard passing TD 7 W 17-3 1 reception for 70 yards 8 L 12-20 46-yard kick return 9 Bye Pro Football Reference

Cohen has always been a happy human, according to his twin. "I don't know what it is about him, but he doesn't let too many things get to him," Tyrell says.

He is happier than ever now, mostly because he appreciates where he is, but you could understand it if he were resentful about where he came from.

His father wasn't a part of his life for most of his childhood. Nor was comfort and stability. His mom worked long hours tending to infirm seniors as a nursing assistant. He was shuffled to six different elementary schools before settling in rural Louisburg, N.C., where he, his mother and three brothers lived in a double-lot trailer with three rooms.

That was then—this is now: Cohen and his teammates go after each other, Lisa Lampanelli-style. Hicks and Co. start with the "short" jokes.

I have an extra car seat for you.

Are you big enough to get on the roller coasters at amusement parks?

Prince Amukamara tells Cohen that a popular team employee who stands about 5'0", "must be your dad."

"Short jokes are weak to me," Cohen says. "Corny."

And so he fires back. His comments can be as sharp as his change-of-direction cuts.

He tells Amukamara he has been wearing the same pair of Adidas sneakers since he was 16.

Long, who is bald, "looks like a bullet."

Quarterback Mike Glennon, 6'6", is "built like a No. 2 pencil. Or a corn dog."

Stacy Revere/Getty Images

Cohen and his teammates also try to get the best of each other with dance-offs and freestyling.

"Rapping might be my next job after football," Cohen says. "When I was younger, I thought it would be. But football was the better career path. I feel if I got serious, I could be something."

Cohen has almost as many nicknames as positions. "Brace Face," which Amukamara called him, was recently retired. Teammates and family members call him "Reek." In college, he earned the nickname "The Human Joystick" for his video game running style.

NFL Network analyst Daniel Jeremiah looked at the way Cohen was able to make something good out of something bad and dubbed him "Chicken Salad." Cohen likes that one. Cohen called himself "Chocolate Badger" as a tribute to Cardinals safety Tyrann "Honey Badger" Mathieu after Mathieu reached out to him earlier in the season. And he also goes by "Big Daddy."

"He tries to carry that Big Daddy name around," Cunningham says. "And the running backs, we are behind him."

They are behind him, that is, as long as they can keep up with him.

Dan Pompei covers the NFL for Bleacher Report. Follow him on Twitter: @danpompei.