For instance, there was a routine fly ball to left field in the top of the seventh inning that seemed like it would easily score D-backs shortstop John McDonald from third base.
And then Harper caught the ball and fired a 300-foot strike to Nats catcher Wilson Ramos.
Frame that moment and savor it. That's why he's different.
"I didn't know he had that kind of arm," Washington pitcher Jordan Zimmermann said.
McDonald scored on the play, but television replays showed it to be very close. Regardless, it made a point.
Earlier in the game, Harper had sprinted near the left-field seats to make a difficult catch on a Gerardo Parra fly ball seem routine.
In those two instances, Harper showed off why he was the No. 1 pick in the 2010 First-Year Player Draft and why a lot of people think he's going to be one of baseball's next great players.
OK, let's push the pause button to make two points.
First, the Nationals would like to remind you that Harper is only 19 years old, that he has played just 129 Minor League games and that there's a reasonably good chance this first tour of the Major Leagues will have some bumps.
Mickey Mantle and Alex Rodriguez were so overwhelmed as young players that both were sent back to the Minor Leagues for a brief time. Cal Ripken began his Hall of Fame career with a 12-for-99 slump.
Regardless of Harper's skills, regardless of his physical and emotional development, this is an incredibly difficult game, and so it would be unfair to put an entire franchise on his shoulders at this point.
And there's something else.
The Nats are 0-3 since Harper entered their lineup, and they've lost five in a row overall with Tuesday's 5-1 decision against the D-backs in front of 22,675 at Nationals Park. Their offense has scored seven runs in the five games, and the lead in the National League East is down to a half-game over the Braves.
So with the media focused on the kid, Washington is searching for answers, and those answers may be tough to come by with the heart of the lineup -- Ryan Zimmerman and Michael Morse -- on the disabled list.
And yet, the Nationals seem to get it. They understand why Harper is in the big leagues at 19, and they understand why there was a mob of reporters surrounding him before and after Tuesday's game.
Something as simple as a round of batting practice became a spectator sport as the baseball rocketed off Harper's bat. His swing is quick, his approach aggressive.
Harper scattered balls over the center-field wall, into the upper deck in right field, here, there and everywhere. In the first inning, he learned he may not see a lot of those batting-practice fastballs during games.
"I was throwing quite a few [changeups]," D-backs starter Trevor Cahill said. "I know he's got a quick bat, so I was just kind of hopefully having him speed it up and pulling the string on it a little bit."
Harper shrugged 'em off.
"I've been seeing offspeed [pitches] my whole life," he said. "I should be able to hit it by now. I chased two pitches I shouldn't have. I was trying to make something happen at that moment."
Harper even made news on Monday's day off when he was coaxed into a slow-pitch softball game during a tour of the city's monuments. Video surfaced of him swinging and missing at one pitch, then lofting a fly to right field as one of the players yelled, "You have to run, rookie!"
Harper laughed about it.
"They see the rat tail and the tattoos. I think they can notice that," he said. "They asked me if I would take a few hacks."
Harper couldn't resist a challenge and seemed to have a blast doing it. Now the goal is to find some normalcy.
"That's what we're trying to do now," Nats first baseman Adam LaRoche said. "We're trying to make him as comfortable as we can with the stuff that's going on. He's part of this team. He's in the lineup. I'm glad he can handle it. He's a guy that knows he belongs here. I don't think all of this is going to faze him."