A cross-section of skeletal muscle in this light micrograph shows the individual, parallel muscle fibers (red). These fibers work in concert to power movement. Thomas Deerinck, NCMIR/ScienceSource hide caption

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Oklahoma Gov. Mary Fallin issues a statement to the media after the execution of Clayton Lockett. Oklahoma Secretary of Safety and Security Michael C. Thompson stands behind her at the Oklahoma State Capitol in Oklahoma City. Alonzo Adams/AP hide caption

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Testosterone levels in men can go up and down throughout the day. Katherine Streeter for NPR hide caption

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Katherine Streeter for NPR Katherine Streeter for NPR hide caption

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False-color transmission electron micrograph of a field of whooping cough bacteria, Bordetella pertussis. A. Barry Dowsett/Science Source hide caption

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There today, here tomorrow: A mother holds her child for a measles vaccination in Manila, Philippines, in January. Travelers are bringing measles from the Philippines to the United States. Noel Celis/AFP/Getty Images hide caption

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