Our feet are resilient. Yes, it's true that people's feet tend to hurt when first starting barefoot running and sharp objects exist in the world. What most barefoot runners find, however, is that the more they do it the less of a problem sharp objects become. This is for three reasons: First, you get better at spotting and avoiding the really nasty stuff that can do significant damage to your feet. You can't cut your foot on a piece of glass if you never step on it in the first place, right? Second, a barefoot runner's feet build up thickened soles with consistent use. The skin doesn't necessarily harden or callous, but it does become more durable and resistant to most of the small rocks and other debris on the ground. Finally, the technique used to run barefoot makes your feet less susceptible to cuts because your feet do less pushing off of the ground. If you were to land on a sharp object you might get poked, but a cut that breaks the skin is not as likely.