Giant pandas at National Zoo

<div class="meta "><span class="caption-text ">In this April 20, 1974 black-and-white file photo, The National Zoo&#39;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&#39;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.  (AP Photo&#47;Charles Tasnadi, File)</span></div>
April 17, 2012 1:03:26 PM PDT