Giant pandas at National Zoo

In this April 20, 1974 black-and-white file photo, The National Zoo's giant pandas, Ling-Ling and Hsing-Hsing play in their yard in Washington. The National Zoo is celebrating 40 years of pandas. Monday was the 40th anniversary of the day pandas Hsing-Hsing and Ling-Ling landed at Andrews Air Force Base in Maryland. The pandas were gifts to the United States from China following President Richard Nixon's historic visit to the country. The pandas were officially presented to the Zoo on April 20, 1972. Ling-Ling lived at the National Zoo until 1992, when she died. Hsing-Hsing died in 1999.  <span class=meta>(AP Photo/Charles Tasnadi, File)</span>
April 17, 2012 1:03:26 PM PDT