Writer Samuel Langhorne Clemens, better known by his pen name Mark Twain, not only redefined American literature in the 1800s; he helped create the very idea of what it meant to be an American. "Sunday Morning" national correspondent Robert Costa visits Twain's boyhood home in Hannibal, Missouri, and sits down with Pulitzer Prize-winning biographer Ron Chernow to discuss his latest book, "Mark Twain," which delves into the life, complexities and sardonic humor of a man who remains a vital presence in American culture.