The sellout crowd at Nationals Park had quieted — as had the home team’s bats — since the Washington Nationals erupted for seven runs in the first inning of Game 4 of the National League Championship Series last Tuesday. The St. Louis Cardinals, facing elimination, had trimmed Washington’s lead to three, and with 12 outs still to get, tension was running high in the sixth inning. Temporary relief came in the form of a children’s song.

When Gerardo Parra was introduced as a pinch hitter, nearly 44,000 people rose, extended their arms and clapped their hands together in unison as “Baby Shark,” the infectious earworm Parra has used as his walk-up song for the past four months, blared over the ballpark’s sound system.

Dave Lundin, the Nationals’ vice president of production and broadcasting, has been with the team for 13 years. He attended Stephen Strasburg’s 14-strikeout debut in 2010 and saw Jayson Werth’s walk-off home run in Game 4 of the NL Division Series two years later, and he believes last week’s “Baby Shark” singalong was the loudest he has heard Nationals Park.

“Everybody did it,” Lundin said. “It’s a special moment now.”

The improbable tale — how an old campfire ditty that was reinvented by Korean children’s entertainment brand Pinkfong in 2015 and grew into a cultural phenomenon last year became the rallying song of D.C.'s first World Series representative in 86 years — begins on June 19.

Washington was 33-38 and 8½ games out of first place in the NL East. Parra, who was signed by the Nationals in May after he was released by the San Francisco Giants, was mired in an 0-for-22 slump and requested a new walk-up song before the first game of a doubleheader at home against the Philadelphia Phillies.

“I think God send to me,” Parra said recently of choosing “Baby Shark,” a nod to his 2-year-old daughter, Aaliyah, instead of another Reggaeton or hip-hop song like he has used throughout most of his 11-year major league career.

There couldn’t have been more than 5,000 people in the stands when Parra stepped to the plate to lead off the second inning that day. Neither the Nationals’ TV nor radio broadcasts mentioned the song, which preceded a routine groundout to first base.

“The day he picked it, my initial response was, ‘Seriously?' ” said Lundin, who, as the father of a 3-year-old, was plenty familiar with the maddeningly catchy tune. “But it just took off. From there, it just got bigger and bigger and bigger.”

“Baby Shark” might have been a forgotten, one-day experiment had Parra not doubled in his next at-bat and homered later in the game, a 6-2 Nationals win, to break out of his slump.

“My girl loves that song,” Parra told reporters afterward. “ … She sings it a lot.”

Parra earned a rare start at home four days later, and this time, MASN’s cameras showed pockets of fans clapping along to the song every time he came to bat.

“Evidently that ‘Baby Shark’ song is sweeping the country,” play-by-play man Bob Carpenter deadpanned.

“I don’t mind it,” analyst F.P. Santangelo replied. “ … I had to listen to Barney with my kids forever, so this is way better.”

Parra was in the lineup again July 4 and went 2 for 4 with a pair of doubles. After he fouled off a pitch in the fifth inning, Nationals Park organist Matthew Van Hoose played his own rendition of the song, prompting Santangelo to declare that “Baby Shark” was officially “becoming a thing.”

“We knew we had something,” Lundin said.

The next day, Lundin’s production team conducted a video shoot with Parra, a routine undertaking with newly acquired players, with the purpose of capturing footage to use on the scoreboard and promotional materials throughout the year. When one of Lundin’s colleagues asked Parra whether he would be willing to do the “Baby Shark” dance during the shoot, the outfielder happily obliged.

The Nationals were six games over .500 and in sole possession of the first NL wild-card spot when they returned from a 10-day road trip after the all-star break to open a series against the Rockies on July 23. As “Baby Shark” began to play after Parra was announced as a pinch hitter in the seventh inning that night, an animated video on the center field scoreboard depicted three images of Parra clapping along in uniform, his head edited to be disproportionately larger than the rest of his body.

“The first time he saw it was when it first played, and he definitely had a smile on his face,” Lundin said of the video, which has been used for Parra’s plate appearances since. “The fans were way into it even before we put that up, but it was just another thing to add to the list of fun.”

Parra followed with a bases-clearing single, prompting a second “Baby Shark” singalong. The video was featured on “Good Morning America” the next day.

“We all thought it was just going to be a game or two or a week, but it turned into something cool,” second baseman Brian Dozier said. “Anything to get the fans involved."

It should be noted that Parra was not the first major leaguer to use “Baby Shark” as his walk-up song. A Venezuelan countryman, Texas Rangers shortstop Elvis Andrus, used the song on Opening Day in honor of his 1-year-old son and continued using it off and on throughout the season. The Nationals, though, took “Baby Shark” and made it their own.

Around the same time Parra’s animated video debuted at Nationals Park, he and his teammates began celebrating hits by making the hand motions from the “Baby Shark” music video. After a single, the base runner turns toward the dugout and gives the baby shark signal by moving his thumb and forefinger together like pincers. Doubles called for the mommy shark, which involves the player clapping his hands with the wrists together. The daddy shark — flat hands and outstretched arms with a clapping motion — was reserved for triples. Nationals players in the dugout began responding by making the appropriate hand motion back.

“Anything that happens in baseball is never planned,” Dozier said. “It’s not like, ‘Hey, you guys want to get a single and let’s do this?’ It’s always something happens: One person does it, and it just catches on."

“A couple of times I’ve come out of a game in the eighth or the ninth inning and I’ve been in the dugout, and I’ve been able to do it to the guy,” Sean Doolittle said of the hand motions, which relievers don’t normally get to experience from the bullpen. “I’ve had a lot of fun with that.”

Opposing players have gotten a kick out of the Nationals’ “Baby Shark” antics, too. When the Braves visited Nationals Park in late July, Atlanta first baseman Freddie Freeman was curious about the hand signals and asked Howie Kendrick to explain.

“It’s ‘Baby Shark,’ ” Kendrick replied with a smile, showing Freeman the hand motion again.

Freeman smiled and nodded as he returned to his position, but judging by his reaction in the seventh inning when Parra came to the plate as a pinch hitter, he wasn’t prepared for his first full-on “Baby Shark” attack. Freeman covered his mouth with his hand to hide his smile as Parra stepped into the batter’s box.

Parra struggled at the plate for much of August and September but caught fire during the Nationals’ final homestand of the regular season and made the playoff roster as a reserve. He had one at-bat at home during the NLDS, and while it came with the Nationals trailing the Los Angeles Dodgers by four runs in Game 3, his appearance led to one of the loudest moments of the night.

In Game 3 of the NLCS, Parra attached a plush blue shark to the netting in front of the Nationals’ dugout. His Instagram story revealed that he had received a box of toys from WowWee, the Canadian company that produces “Baby Shark” toys for Pinkfong. The plush shark was back for Game 4, and in the sixth inning, it brought Parra good luck after fans, some dressed in shark costumes and wearing “Parra Shark” T-shirts, bandannas and hats, serenaded him with the loudest “Baby Shark” singalong yet.

“Of course he was going to get a hit,” analyst Ron Darling said on TBS’s broadcast after Parra delivered a two-out single.

“I only put him in the game today to get the fans going again,” Nationals Manager Dave Martinez joked after Washington completed the sweep with a 7-4 win.

“Baby Shark” euphoria continues to spread throughout the District. On Friday, Nationals players attended a Capitals game and joined the entire arena in chomping along to “Baby Shark” during a stoppage in play. Costumed members of the National Symphony Orchestra performed a unique rendition of the song at the Kennedy Center on Sunday.

Parra, who plans to get a “Baby Shark” tattoo if the Nationals win the World Series, spent part of his weekend at Children’s National Hospital, meeting with pediatric cancer patients and handing out plush baby sharks and Nationals hats.

“It’s amazing, the energy the fans bring to me and to my teammates,” Parra said. “But I think the more important [thing] is all the kids jumping, clapping, enjoy that. It’s great when I see other guys do that. … Right now, it’s amazing. It’s working for us."

As the Nationals prepare to host the city’s first World Series game since 1933 on Friday, Lundin said there are no plans to alter the “Baby Shark” routine.

“We’re just rolling with what’s working,” he said. “It’s what’s got us here. We’re not trying to change what the fans are doing or what Parra’s doing or anyone else. We just want to help enhance it. It’s been a great ride.”

Sam Fortier and Elahe Izadi contributed to this report.

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