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http://www.eggsafety.org/consumers/c...r-faqs#Safety4
An egg can float in water when its air cell has enlarged significantly to keep it buoyant. This means the egg is old, but it may be perfectly safe to use. Crack the egg into a bowl and examine it for an off-odor or unusual appearance before deciding to use or discard it. A spoiled egg will have an unpleasant odor when you break open the shell, either when raw or cooked.