– Brad Childress remembers the Chiefs stealing a play from North Dakota State four years ago in a series of moves that helps explain why his friend Andy Reid has remained a new-age mind in an old-school body.

“I guess I shouldn’t say ‘stolen,’ so let’s say it’s a play we ‘researched,’ ” laughed Childress, the former longtime Reid assistant and Vikings head coach from 2006-10.

“We were studying Carson Wentz before the draft. I said to Andy, ‘Look at this play.’ We ended up making a cut of it because it fit us. We repped it in the springtime and were able to call it three or four times that year.”

The play included a jet sweep with four vertical routes to the short side of the field.

“Andy has always been amenable to new ideas, and this was when the NFL was really just starting to take the college jet sweep seriously,” Childress said. “He comes from the West Coast offense, but the offense is ever-changing with Andy in charge.”

That creativity is a big reason Reid is sixth in coaching wins in NFL history at 207-128-1. And that 21-year track record coupled with an 0-1 Super Bowl mark is why this 61-year-old, average-Joe-looking fan favorite is the sentimental choice in Sunday’s showdown with 40-year-old Kyle Shanahan’s 49ers.

“We absolutely borrow things,” said Reid, who’s back in the Super Bowl for the first time in 15 years. “If it’s good, we’re going to sneak it from you, put our own spin on it and then claim it. Right? That’s how coaches do it. But, yeah, we snuck a couple of things from Carson’s offense.”

Shanahan has the old-school running game and the better defense. But the old man has the cutting-edge offense and Patrick Mahomes, the electrifying young quarterback that Reid and Brett Veach, the general manager Reid hired, projected as an NFL superstar long before anyone else.

“One thing you have to keep in mind working for Coach Reid is to remain flexible,” said Eric Bieniemy, the Chiefs’ offensive coordinator and former Vikings running backs coach. “Just because we know things a certain way, they aren’t etched in stone around here. It’s also about being creative and forward-thinking. We want to stay two steps ahead.”

Childress was 30 years old and working as offensive coordinator at Northern Arizona when the school needed someone to fix its broken offensive line in 1986. They hired Reid, then 28, away from San Francisco State. He stayed only one year before moving on to UTEP, but he and Childress hit it off and were reunited in Philadelphia when Reid got the Eagles’ head coaching job in 1999.

By that point, Reid had spent eight seasons in Mike Holmgren’s rigid West Coast scheme in Green Bay. Holmgren, a true West Coast loyalist as a first-generation disciple of Bill Walsh, wasn’t interested in fresh ideas from what Childress describes as Reid’s “A Beautiful Mind” doodling.

“Andy is much different than I would say a Jon Gruden or a Mike Holmgren is,” Childress said. “When he was younger, Andy would bring a play to Holmgren and say, ‘Coach, this is really a good play. Here’s how it works and da, da, da, da, da.’

“Holmgren would say, ‘You know what, that probably is a pretty good play. And you know what else, when you get your head coaching job, put that in your book and you can run it in your offense. But I don’t know that play so I’m not running it.’ So that’s kind of the anti-Andy.”

Reid seems to have almost as many supporters from Philadelphia, where he was fired on New Year’s Eve in 2012, as he does in Kansas City, where he was hired seven days after his firing.

“I enjoyed my time in Philadelphia,” Reid said. “I think what happened was good for both sides. It was good for them, and it was good for me.”

At the beginning of the Eagles’ 2012 season, Garrett Reid, Andy’s oldest son and training camp assistant, was found dead of a heroin overdose in his training camp room. Today, Reid’s son Britt, who also battled heroin addiction, is a Chiefs linebackers coach.

Childress said the fresh start in Kansas City got Reid back to his coaching roots. He got back to being hands-on in the weekly game plan installation and turned the bulk of the personnel duties over to his front-office people.

“When he got to Kansas City, he made a point to go, ‘You know what, why I got into this business was to coach,’ ” Childress said. “That’s what he loves to do.”

In his office, Reid has a giant white board that holds every idea imaginable for that week’s game plan.

“By the end of the week, it really is the ‘Beautiful Mind’ board with plays written on top of plays,” Childress said.

“And,” said Bieniemy, “by Saturday, every color that is known to man is used on that board.”

And every week, Reid takes a picture of that board and puts it in a three-ring binder before the board is wiped clean to begin anew.

“You never know when you might need one of those ideas,” Bieniemy said. “Coach Reid tells us not to take anything personal. If an idea gets shot down, don’t worry about it. Just keep those ideas coming. That’s Andy.”