human urine is both nutrient-rich and an effective tooth whitener

Endurance, after all, is the only reason we even exist. We think of ourselves as nature's deadliest animals, but the truth is, a naked human is the biggest wimp in the wild. We have no fangs, no claws, no strength, and no speed.

I have the idea that running shoes are based on a kind of cult idea - that our feet are flawed and we need shoes to correct those flaws. The shoe companies are in the business of selling shoes. But there's no evidence from running shoe manufacturers that they're right. There's no scientific data that running shoes reduce injury.

In 'Tarahumara' land, there was no crime, war or theft. There was no corruption, obesity, drug addiction, greed, wife-beating, child abuse, heart disease, high blood pressure, or carbon emissions. They didn't get diabetes, or depressed, or even old: 50-year-olds outran teenagers.

Deny your nature, and it will erupt in some other, uglier way.

The words of the social critic Eric Hoffer were ringing true: "Every great cause begins as a movement, becomes a business, and turns into a racket."

There's something so universal about that sensation, the way running unites our two most primal impulses: fear and pleasure. We run when we're scared, we run when we're ecstatic, we run away from our problems and run around for a good time.