Now, more than ever, a diversifying United States needs a shared base of knowledge. That’s according to Eric Liu, executive director of the Citizenship and American Identity Program at the Aspen Institute. He’s calling on the American public and cultural leaders to build a crowd-sourced national list of facts and references every American should know. In this “Extra” episode, Liu talks to Colin Woodard, an award-winning author and journalist. In his book, “American Nations: A History of the Eleven Rival Regional Cultures of North America,” Woodard describes why American values differ across the country. (Music: David Szesztay/Throughout The City)