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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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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A study that measured antibodies and the disease history of more than  200 kids found that two doses of vaccine were 98 percent effective in  preventing chickenpox compared with 86 percent for a single dose. AP hide caption

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