Entrepreneurs love to say yes—to new ideas, to new opportunities, and to new markets. But yes can be dangerous: The wrong yes will compound itself, stretching a company thin and clouding its sense of purpose. Hamilton Powell learned this the hard way: As he built his watch company Crown & Caliber, he said yes so often that he was soon working 100-hour weeks and burning out his employees. Then he hired a COO who trained as a drill sergeant—and everything changed. Here’s how Hamilton learned to say no.