Maria Fabrizio for NPR

This fungus among us — baker's yeast, aka Saccharomyces cerevisiae — is useful for more than just making bread. iStockphoto hide caption

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People exercise along the Reflecting Pool on the National Mall in Washington, D.C.; the metro area was ranked as America's fittest city for the second year in a row. Andrew Harnik/The Washington Times/Landov hide caption

itoggle caption Andrew Harnik/The Washington Times/Landov

Sam Swiller and his dog, Sully, in their home in Washington, D.C. Meredith Rizzo/NPR hide caption

itoggle caption Meredith Rizzo/NPR
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