As America turns 250, many celebrations are looking back on our nation's past – but one looks to the future. By law, a time capsule is to be buried in Philadelphia on July 4, not to be opened until 2276, in time for America's quincentennial. Correspondent Faith Salie looks into what objects – high tech and low – made the cut to be preserved for posterity; and how the capsule itself was designed to survive 250 years underground.