At age 21, climber Tommy Caldwell pushed his kidnapper off a cliff. Here's what he learned from the experience.

National Geographic/Taylor Shaffer; photo courtesy of "The Devil's Climb"/Greg Child

In "The Devil's Climb," a National Geographic documentary, famed rock climbers Tommy Caldwell and Alex Honnold ("Free Solo") break a world record by climbing a treacherous Alaskan mountain in under 12 hours. At one point in the film, Caldwell talked about how dealing with past adversity helped him persevere through an Achilles injury.