Not all young girls avoid dirt. Hannah Rose Akerley, 7, plays in a gigantic lake of mud at the annual Mud Day event in Westland, Mich., last July. Bill Pugliano/Getty Images hide caption

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First lady Michelle Obama and Bill Simon, president and CEO of Wal-Mart's U.S. unit, at a media briefing on the company's food initiative held in Washington, D.C. Mark Wilson/Getty Images hide caption

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Domestic fowl, like chickens, often are carriers for the new flu viruses that cause human outbreaks. David Stephenson/ hide caption

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Antonio Garcia re-stocks the beverages at The Corner Market in Washington, D.C., in 2010. Study author Jean Welsh says that sweetened beverages tend to be the biggest source of added sugar. Jacquelyn Martin/AP hide caption

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A Dutch man rides his bike by windmills in Kinderdijk, the Netherlands in 2007. Dutch researchers have found that the ability to ride a bike can help doctors distinguish between two similar forms of Parkinson's disease. Evert-jan Daniels/AP hide caption

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