The float test is the most reliable home method for checking egg freshness. Place the uncracked egg in a bowl or glass of cold water. A fresh egg sinks and lies flat on its side. A week-old egg sinks but tilts upward at an angle. An older egg stands on one end underwater. A floating egg should be discarded — the air cell inside has grown large enough to make the egg buoyant, and the contents have started to break down.
The science: eggshells are porous, so moisture and carbon dioxide slowly escape and air seeps in. Over weeks, the air cell at the wide end of the egg grows from a small pocket to a large bubble. The bigger the air cell, the more the egg floats, and the older the egg.
There are two other quick checks. Crack the egg onto a flat plate — a fresh egg has a firm, compact yolk and thick white that stays close to the yolk. A watery white that spreads wide on the plate is a sign of age. And your nose is definitive: a bad egg smells unmistakably sulfuric, and you should not cook or eat it.
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