President Obama traveled in the presidential limo with his dog, Bo. Adviser David Axelrod tweeted this photo from the White House Flickr account to tweak the Romney campaign. Pete Souza/White House hide caption

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Republican presidential hopefuls former House Speaker Newt Gingrich and former Massachusetts Gov. Mitt Romney debate in Jacksonville, Fla., on Thursday. Matt Rourke/AP hide caption

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Introverts, who prefer quieter, lower-stimulation environments, have trouble thriving in today's extrovert-oriented culture, says author Susan Cain. hide caption

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