This essay first appeared inWhy Is the Penis Shaped Like That? And Other Reflections on Being Human, published by Scientific American/Farrar, Straus and Giroux, 2012. Once, while in a drowsy, altitude-induced delirium thirty-five thousand feet somewhere over Iceland, I groped mindlessly for the cozy blue blanket poking out beneath my seat, only to realize—to my unutterable horror—that I was in fact tugging soundly on a wriggling, sock-covered big toe.