Every day, celebrities, politicians, moms and dads log on to answer the question, "What are you doing?" The answer is called a "tweet," and it can be just about anything…as long as it's short. Posts are limited to 140 characters or less.
Users post everything from food diaries and marriage proposals to breaking news. In January 2009, a Twitterer was the first to post a photo of the U.S. Airways plane that crash-landed into the Hudson River.
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