Jennifer Aniston: I'm Jealous of Nicole Kidman's "8-Pack" Abs

Credit: James Devaney/; Steve Granitz/

Jennifer Aniston's got some ab envy!

While shooting the rom-com Just Go With It with Nicole Kidman in Hawaii, the 42-year-old actress said it was hard to believe how "rock-solid" the mom of four's midsection was.

"I walked up to her and I said, 'I'm sorry, can I touch them? I have to touch them,'" Aniston revealed on Ryan Seacrest's KIIS-FM radio show Friday. "'Is that spray-painted? What is that?' I think it's an eight-pack."

The single star -- who's been known to flaunt her own toned tummy in skimpy two-pieces over the years -- said she and Kidman, 43, even became workout buddies off set.

"We did yoga a couple of times out there in Maui. We did some planks together," Aniston said. "Yoga is all core."

As for how she maintains her own six-pack? "It's just sit-ups!"