Can you believe she's 40-years-old?!

Heidi Klum covers the March issue of Women's Health, and the supermodel not only shows off her amazing midriff, but she tells the magazine how she achieves such a hot bod!

    "If it's [exercising] with my kids or we're going on a hike with the dogs or jumping in the swimming pool, or we have a gigantic trampoline in the backyard... Every once in a while I go on the treadmill, but I don't love that so much. I like the outdoors a lot," she explains of her workout routine preferences.

The former Victoria's Secret stunner admits it's harder to stay in shape as you get older, too.

    "The metabolism definitely changes when you turn 40. I always thought, 'That's not going to happen to me', but it is happening to me. If I indulge more, I have to exercise more. I figured out what works for me," she confesses.

So does the German beauty ever give into her cravings?

    I'll eat everything but the pasta and bread. Do I want pasta? I do. But I choose not to eat it. [I was] out to dinner this weekend, and truffles are in season. I had to have the truffle pizza and the truffle pasta. It's not like I don't [indulge], but not all the time," she reveals.

Whatever she's doing, it's working!