A pair of Denver Zoo lion cubs are finding enrichment and strength training from an object they probably wouldn't find in the wild -- a giant steel ball.

The Denver Zoo, which posted video of the cubs playing on YouTube, said the large steel ball suspended from the ceiling of Kalu and Kamara's indoor enclosure encourages the lions to stretch to reach it and strengthen their muscles by trying to move it.

The zoo said the ball is meant "to encourage natural behavior, like playing."

Kalu and Kamara made their public debut in November, two months after they were born at the zoo.