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As an egg ages, its internal air cell gradually becomes larger.
That's why an older egg may tilt upward or float when placed in water, while a relatively fresh egg tends to stay lower.
However, the float test indicates age, not food safety. A floating egg isn't automatically spoiled, and a sinking egg isn't guaranteed safe.
If you're uncertain, crack the egg separately and evaluate it appropriately rather than relying on the water test alone.
© USDA