Bill Appel has devoted his retirement years to helping strangers.

He’s a “trail angel,” providing support to hikers and mountain bikers on several long-distance trails. He offers food and beverages to travelers, gives them rides into town to resupply, and cheers them on at some of the most demoralizing points in their journeys.

It’s a year-round operation. Appel angels along the Appalachian Trail, the Continental Divide Trail, the Colorado Trail, and the Florida Trail — and he does it all for free.

On this episode, we pay a visit to one of his aid stations, and we explore why people are sometimes so selfless. Where does altruism come from? What makes a person commit repeated acts of kindness?

FYI: the answer is not what you’d expect.