Amber McGowan, left, and Krystal Klacsan work Monday at Cannabis City in Seattle, a day before the store is to begin legal pot sales on Tuesday. The store will be for now the only one in Seattle to sell recreational marijuana. Ted S. Warren/AP hide caption

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People smoke marijuana, presumably, because it affects their brains, not despite that fact. Above, people in Sao Paulo, Brazil, campaign for the legalization of marijuana. Nelson Almeida/AFP/Getty Images hide caption

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Former Supreme Court Associate Justice John Paul Stevens likens making pot illegal to Prohibition. In his new book, Six Amendments, he proposes constitutional changes including a curb on an individual's right to bear arms. Chip Somodevilla/Getty Images hide caption

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With the Colorado state capitol in the background, Cannabis Cup attendees dance and smoke pot at the annual 4/20 marijuana festival in Denver. Brennan Linsley/AP hide caption

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The teenage years are the last golden opportunity to build a healthy brain, researchers say. So smoking pot might not be so smart. Tomas Rodriguez/Corbis hide caption

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Research in mice offers new clues as to why Harold and Kumar were so motivated to get to White Castle. Todd Plitt/Getty Images hide caption

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Truffles are among the many foods infused with THC – the chemical in marijuana that gives you a high — already for sale in Colorado. Luke Runyon/KUNC/Harvest Public Media hide caption

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