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Ashton Kutcher

Ashton Kutcher is really proud of a paparazzi shot of him showing off his butt crack, because it was taken during a good weight week.

As the 40-year-old actor co-hosted The Ellen DeGeneres Show on Tuesday, Ellen asked him to explain why he's so proud of a snap which shows him with his jeans falling down to expose the top of his rear.

And detailing why the shot shows him he's having a "great week," Ashton elaborated: "I fluctuate by about 10 pounds, I got a 10-pound flux that happens. So I'm either ripping it at 198 or 188, or somewhere in between. So when the pants are coming down, that means that's a 188 week - I'm feeling great!"

Laughing, he added: "A gift for me, a gift for you - it's the holidays!"

Elsewhere in the interview, Ellen - who is neighbours with Ashton, his wife Mila Kunis and their two children - ribbed the actor for always carrying two-year-old son Dimitri when the family are out and about.

"Ashton and Mila (are) so cute, they walk in the morning with their pajamas on, and their dogs, and their kids. And the young boy that never walks is how old?" she asked. "He never puts him down. He's always on his back."

"OK, wait, I now have a complex about this because I have one of those hiking backpack things that I put him in to go down to the beach so that we can actually go down the beach," Ashton hit back. Otherwise it's like a whole wrangling session with the kid."

"Then you came out, and you were like, "Oh, does he walk?' and I was like, "Yes, he walks,'" Ashton went on. "So now I take him out of the thing once we get to (view) of your house and I'm like, "Go, go, go! Run, little sucker!' And I'm like trying to dish him along so I don't have to hear about it anymore. This kid's gonna be an Olympian, man, 'cause I'm gonna be behind him like, "To Ellen"s house! She's watching!'"