Photo: Bryan Bedder, Getty Images

Lots of celebs are running marathons for worthy causes -- Ryan Reynolds did it, and Edward Norton is training for one, too. Jessica Biel is also charity-minded, says FitSugar, but this star is taking a different path -- up a mountain. Yep, she'll climb Mt. Kilimanjaro (the tallest, free-standing mountain in the world) for the Summit on the Summit, a group dedicated to educating the public about the need for clean drinking water worldwide.

Why a mountain climb? "This is a basic human necessity that needs to be addressed now," says Biel. "[I want] to help any way I can in order to raise awareness toward the life-threatening clean-water crisis happening not only in Africa but around the world."

Think mountain climbing is too much to tackle? Let Zak York inspire you. After surviving childhood cancer, he took on Mount Whitney, the tallest mountain in the continental U.S., and he plans to do it again.