MILWAUKEE -- Calling a grand slam is one thing. Calling a grand slam when the bases are empty is another.

Add it to the growing legend of Brewers rookie Jackson Chourio, for whom nothing seems impossible these days.

“Crazy,” said Brewers veteran starter Frankie Montas. “Insane.”

Montas and Chourio’s other Brewers teammates swear the following story is true. As the Brewers came off the field in the middle of the sixth inning of Monday’s 9-3 win over the Cardinals, with home runs from Willy Adames and Rhys Hoskins already on the board and a sellout crowd of 41,731 munching on $1 hot dogs, Chourio approached Montas and some others in the dugout.

“I’m going to get an at-bat with the bases loaded,” Chourio said, “and I’m going to hit a grand slam.”

Asked about that later, Chourio said, “I think I was just messing with them.”

But that’s exactly what he did. Hoskins walked, Sal Frelick doubled and Joey Ortiz walked before Brice Turang saw Paul Goldschmidt rob him of a run-scoring hit. That served to keep the bases loaded for Chourio, who deposited a slider from Cardinals reliever Riley O’Brien into the left-field bleachers for his 18th home run of the season and his second career grand slam.

Eight players have hit multiple grand slams in the big leagues before turning 21 years old, including Ted Williams, Mickey Mantle, Alex Rodriguez -- and Chourio. It’s unclear whether any of the others called any of those slams before a single runner was on base.

“It was incredible,” Adames said. “That’s why he was pointing to me and Frankie. We were talking about it and he was like, ‘I’m going to hit it.’ And he did it. I was like, ‘Oh my God, he really did it.’ …

“He’s showing he’s superstar material to the world.”

Added Monday’s Brewers starter, Freddy Peralta: “And he told me again, ‘You see how good I am?’”

Chourio has asked that question of Peralta several times now.

Each time, Peralta just laughs.

“I just say, ‘I know you’re great,’” Peralta said.

Signed to an eight-year, $82 million contract in December that represented a record for a player with as many days in the big leagues as the Brewers had on base when Chourio predicted his grand slam, the Venezuelan slumped so badly in May that it took what amounted to an intervention from Adames, Peralta and William Contreras to lift him back up.

Since the start of June, Chourio has been a different player. He’s slashing .320/.374/.544 since June 2, with 13 home runs and 13 stolen bases, pushing him past 20 steals for the season and giving him 18 total homers. With two more, he would be the third player in AL/NL history (along with Vada Pinson and Mike Trout) to have a 20/20 campaign in his age-20 season, and the first to do so before his 21st birthday, as Chourio won’t turn 21 until next March.

“I mean, he’s in that sweet spot,” Brewers manager Pat Murphy said. “He’s got just enough smile and just enough intensity, just enough focus. It seems like he’s born to do this.”

Murphy didn’t want it overlooked that Chourio also walked three times, and wondered how many players aged 20 or younger have ever walked three times and hit a grand slam in a Major League game.

“I bet there’s under 20, in all those games,” Murphy said.

How about zero? Before Chourio, only six times since 1901 had a player walked at least three times and homered in a game before his 21st birthday. None of those homers were grand slams. The last to do it was Washington Senators outfielder Jeff Burroughs in 1971. Before that? White Sox third baseman Bob Kennedy in 1940.

“The kid, it’s the right decibel, or the right frequency, whatever it is,” Murphy said.

Thankfully, it didn’t all come to an end on Friday in Cincinnati. In Game 1 of the Brewers’ doubleheader sweep of the Reds, Chourio stepped on first base and badly turned his right ankle. He needed help from two members of Milwaukee’s athletic training staff to get back to the dugout. Murphy thought for a moment that Chourio’s season was over.

And then he sprinted back to left field at the start of the next half-inning.

“We have a new nickname for him. He’s a soccer player now,” Adames said.

Chourio is willing to wear the good-natured ribbing since it means he gets to keep swinging the bat.

“It’s just incredible,” Chourio said. “It’s just fantastic.”