Zac Efron doesn't just get his rock hard body from the High School Musical choreography -- the actor earns those abs on a surfboard. As a matter of fact, his High School Musical co-star Ashley Tisdale gave him an Indo Board for his 21st birthday to help him improve his surfing skills.

As Zac probably is well aware, activities that keep you off-balance do a phenomenal job at working your core, thus helping to create enviable abs. Surfing is great for that, but lots of other sports provide a tough core workout, too, like water skiing, snowboarding, and skateboarding.

Zac isn't the only celebrity to embrace surfing as a sport -- check out the gallery below to see some other famous folks who know how to catch a wave.