ADDS PRESIDENT HOLDS A MODEL USED TO SHOW HOW POLYMERS EXPAND - President Barack Obama holds a Model used to show how polymers expand as he learns how sandless sandbags work that are the invention of Peyton Robertson, 12, of Ft. Lauderdale, Fla., while touring the 2014 White House Science Fair exhibits on display in the State Dining Room of the White House in Washington, Tuesday, May 27, 2014. Robertson designed a new kind of sandbag to protect against flooding from hurricanes and other disasters. Obama was celebrating the student winners of a broad range of science, technology, engineering and math (STEM) competitions from across the country. (AP Photo/Susan Walsh)
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