WASHINGTON, DC - SEPTEMBER 01:  A clock that was knocked from a wall at the Pentagon during the September 11, 2001 attacks is displayed alongside other artifacts as part of an exhibit at the Smithsonian National Museum of American History September 1, 2011 in Washington, DC. For nine days leading up to the 10th anniversary of the terrorist attacks, the Museum will display more than 50 objects from the World Trade Center, Pentagon and Shanksville, Pennsylvania, in an exhibit titled, "September 11: Remembrance and Reflection." (Photo by Win McNamee/Getty Images)