When Nicole Richie needs to tone up, she turns to Jennifer Lopez -- her music, anyway. The socialite mother of two, who is married to Good Charlotte's Joel Madden, works out regularly to J. Lo's hit new single, 'On the Floor.'

We say 'work out' lightly, because it seems to be more of a dance routine than a dreaded gym session. With her trainer, Tracy Anderson -- who helps keep big names like Madonna and Gwyneth Paltrow in shape -- Richie pumps it to J. Lo to shed those extra pounds.

Apparently, because she danced as a kid (growing up with dad Lionel Richie, no less!) and can eat whatever she wants, Richie really digs Anderson's class. "The dancing is so much fun for me," she says. "This is the only workout that I've ever done that I've been able to eat whatever I want. I feel so great after I do it. I'm in there two hours a day, four days a week. It's so fun and the time flies by. The music is great and I love the cardio."

Anderson -- a self-proclaimed J. Lo fan -- sings both Lopez's praises and Richie's. "The girl can dance! She's a rock star," she says.

Of course, it's easy to rock it when you've got the right soundtrack!

Watch Nicole Richie Work It to J. Lo's 'On the Floor'