Chris Hemsworth’s twin sons couldn’t be more different, with the actor calling one of them “a beefy little kid” and joking the other is “much smarter”

Chris Hemsworth‘s tough-guy skills from his days on the Thor set seem to be rubbing off on his kids — well, one of them, at the very least.

The 12 Strong star dropped by Jimmy Kimmel Live! Wednesday night, recounting an incident where one of his twin sons (Tristan and Sasha, 3½) fractured his femur while riding in a golf cart with the actor, mom Elsa Pataky, his brother and his sister India Rose, 5½, on the Avengers: Infinity War set.

“My son is sitting in the backseat … all of a sudden as they’re driving along, he just leaps out the side of the golf cart, while it’s moving,” Hemsworth, 34, tells Jimmy Kimmel. “He’s half crying and sulking, but more embarrassed that he didn’t stick the landing.”

“[My wife] said, ‘I got to take him to the doctor.’ … The set nurse has a look and says, ‘Look, it’s not broken, he’s fine, he’s not even crying, but take him anyway,’ ” adds the star, describing his fearless son as “a beefy little kid.”

Want all the latest pregnancy and birth announcements, plus celebrity mom blogs? Click here to get those and more in the PEOPLE Babies newsletter.

RELATED VIDEO: Chris Hemsworth and Mark Ruffalo Talk Bringing Their Kids to Thor Set: “It Was a Family Affair!”

When the little boy’s X-ray results came in, his parents were in for quite a surprise. Recalls Hemsworth of the doctor’s reaction, ” ‘Oh my God, it’s a really bad fracture through his femur and is close to breaking all the way through.’ ”

Once his cast came off four weeks later, though, all bets were off. “A day or two later, I’m cooking dinner and he’s standing over on the bench top and he’s like, ‘Papa!’ and I’m like, ‘Yeah?’ and he’s like, ‘Look, my leg’s better!’ ” he says.

“I’m like, ‘No!’ He leaps off the bench — perfect landing,” Hemsworth praises.

christ-hemsworth-1

RANDY HOLMES/ABC

Chris Hemsworth and Jimmy Kimmel

FROM PEOPLETV: The Marvel Cinematic Universe Explained by Shirley Li’s Nerd Map

The Ghostbusters star also addresses a recent incident where the same son scaled the family’s fridge for sweets in a cupboard above, explaining it was a bit of a last resort after earlier strategies for using furniture to climb up failed.

“Nothing stops him,” says Hemsworth, joking that his other son is “much smarter, but doesn’t quite have the athletic ability” his twin does.

“When he runs, it’s like Usain Bolt. When his brother runs, it’s like he’s got flippers on,” the father of three says with a laugh. “It’s so cute. They’ll be the unstoppable pair.”

12 Strong premieres nationwide Jan. 19.