United States' swimmer Michael Phelps smiles as he wars his 1th gold medal at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, Saturday, Aug. 4, 2012. Phelps pushed the United States in front to win the medley relay Saturday in the final swimming event of the London Games, after which he is retiring.(AP Photo/Mark J. Terrill)

Photo by: Mark J. Terrill

United States' swimmer Michael Phelps smiles as he wars his 1th gold medal at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, Saturday, Aug. 4, 2012. Phelps pushed the United States in front to win the medley relay Saturday in the final swimming event of the London Games, after which he is retiring.(AP Photo/Mark J. Terrill)