Two of the Denver Broncos’ newest wide receivers can run.

“They can fly,” wide receiver Demaryius Thomas said of rookie wide receivers Isaiah McKenzie and Carlos Henderson. “They both can run and catch the ball. They can do things after the catch. I look forward to seeing what they can do for the team.”

But just like anything else in life, they’ll need to learn how to walk before they really start to run.

The Broncos wrapped up their third OTA practice on Thursday, and while the talents of the rookie wide receivers didn’t go unnoticed, they must overcome the hurdle that is learning offensive coordinator’s Mike McCoy‘s system before they are really ready to run.

“Number one is learn the system, because they’re swimming,” McCoy said on Thursday. “All the rookies that just got here not long ago, they’re trying to figure out which was is right and left right now. If you were out there at practice the past couple days, you could see the playmaking ability that they have. They’re explosive players.”

Explosive is right.

McKenzie, the Broncos’ second selection of the fifth round, was a top performer in the three cone drill with a time of 6.64, the second fastest of the combine. During his career at Georgia, McKenzie scored just about any way he could. His four rushing, seven receiving, one kickoff return and five punt return touchdowns made up his 17 career touchdowns in his three seasons with the Bulldogs.

Henderson, the Broncos’ third round selection, was no stranger to the end zone either. The 2016 Conference USA Player of the Year scored 33 total touchdowns in his three seasons at Louisiana Tech.

One player on the Broncos’ roster knows first hand just how tough learning a new system can be. Demaryius Thomas went through the same thing with McCoy in 2010, his rookie season.

“I understand totally what they’re going through,” Thomas said. “I couldn’t even line up my first time running this offense. I still remember the day that [former head coach Josh] McDaniels came to Atlanta and quizzed me on it. It was tough.”

“That’s what I tell all the players,” McCoy said. “Not just those two young receivers, but everybody. ‘Learn the system. When you leave this office, you have to study.’ That’s the most important thing, because they’re talented players. Once they learn it and they know where there going and how to get there, it’s going to be fun playing with them.”

As if learning a new system wasn’t hard enough, Henderson and McKenzie have the pleasure of doing it against a stout Broncos defense.

“The great thing for our football team is playing against our defense every day,” McCoy said. “It’s one of the best, if not, the best in the league. Having played against them the past four years, twice a year and then the first year, a third time, it’s great to go against the best in the business.”

Despite the challenges of learning a new system and facing a tough defense, the rookies are getting the hang of things.

“It’s going to take some time,” Thomas said. “But they’re picking it up kind of quick.”