More than 10,000 spies are believed to be residing in Washington, D.C., living among a population of about 700,000 people. That means one in every 70 people you run into in D.C. is likely a spy. How do you tell? What are they up to? How do they do their work? Many of the answers and the profound history of espionage lie inside a new glass and steel structure called the International Spy Museum.