Nicole Richie Credit: Instagram

Fun in the sun, indeed!

Nicole Richie shared a picture of herself paddle boarding via Instagram Jan. 8. Wearing a blue bikini, the 31-year-old Winter Kate designer used her core strength to stay above the waves.

Richie -- once criticized for her pin-thin frame -- credits her toned, healthy physique to pro Tracy Anderson's Dance Cardio classes. "When I first came to a class, I heard I was going to be there for an hour and a half. I was like, 'Oh, I don't do that,'" the Simple Life star said in 2011. "And now I'm in there two hours a day, four days a week. It's so fun and the time flies by and the music is great."

The Fashion Star mentor (mom to daughter Harlow, 4, and son Sparrow, 3, with husband Joel Madden, 33) said it's easy to "find time" to work out because "it's so fun. I look forward to it every day."

Said Richie, "This is the only workout that I've ever done that I'v been able to eat whatever I want. I feel so great after I do it. The dancing is my favorite part."

The California native is also a fan of Pressed Juicery, which delivers its vitamin and mineral-packed concoctions to customers' homes. Of course, Richie maintains a sense of humor about the many ways in which she says healthy. "Telling people you're on a juice cleanse is a perfect way to make sure everyone is visualizing you having diarrhea," she tweeted Dec. 30.