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United Airlines said today that it will require obese travelers to buy a second seat on flights that are especially full and in cases where other passengers complain about being cramped. And how do you know if you're too fat to fly? "Travelers must be able to put the arm rest between seats down to its normal position and buckle a seat belt with one extension belt." There you have it: When you start creeping into one extension belt territory, it's probably time to clip that Xenadrine ad out of the in-flight magazine. [Bloomberg]