“Barry,” my cousin Mike said, “I think it’s time.” It was clear that my brother didn’t feel the same way, but he only shrugged, which Mike took as agreement. “Dave,” he said, giving the words as much gravity as he could muster, “Go get the dictionary.” I was nine years old, and a tag-along. I’d walked in on my brother telling a story about how—during school that morning—a girl he knew got her period in the middle of French class. And I laughed like the dickens. And then they called me on it. After I’d lugged the dictionary down from the spare room, Mike told me to look up the word period and read out the definition. “The end of a cycle, a series of events, or a single action?” “Keep going,” he said. “The full pause with which a sentence closes?” “Not that.” “An interval of geologic—“ “Gimme that!” He yanked the book towards him, read down the page, and pointed me towards the definition he’d found. Menstruation: the monthly discharge of blood from the uterus of nonpregnant women from pu...