Michael B. Jordan and Lupita Nyong’o are not only “Black Panther” co-stars, they’re also fierce competitors engaged in an ongoing — and completely wacky — pushup challenge.

As the “Creed” star explained to Jimmy Fallon on Friday’s “Tonight Show”, he and Nyong’o came up with the idea while travelling for press junkets.

“We’re on these long press tours, we try to keep things fresh and keep things fun, and we’re shooting all these films for months at a time, so we started betting on anything,” Jordan told Fallon.

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That could include, say, “who can throw this piece of paper into the trash,” he added.

“If you miss it, you give me five pushups — but they’re on retainer,” he said. “So that means I can call you up whenever I want and you have to give me a pushup.”

The whole point of keeping the pushups “on retainer,” he explained, is to try to catch the other person at the most inconvenient times, such as while they’re stuck in traffic or caught outside in the rain.

RELATED: Michael B. Jordan Admits He Was ‘Seeing Stars’ After Being Punched On ‘Creed’ Set: ‘It Felt Like I Was In A Car Accident’

For example, Jordan revealed how Nyong’o asked him to give him an owed pushup while they were attending an interactive art display at a fashion show.

“We were pretty much knee-high deep in popcorn,” he said. “She’s a fast learner, and I had to get down there and give her a pushup.”

Later in the show, Jordan and Fallon played the Chrissy Teigen-inspired game “Pour It Out”, in which they take turns providing answers to very personal questions that only they know; if either of them refuses to reveal the question, they must take a shot of a mystery liquid. Watch:

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