Why do some eggs float in water?

A freshly laid egg has almost no air pocket. It comes out at a little over 40°C, and as it cools, the contents shrink more than the shell, forming a small air pocket between two membranes at the blunt end. The whole egg weighs more than the same volume of water, so it settles gently on the bottom of the bowl. An egg lying still on its side usually does not yet contain much air.

As days pass, moisture and carbon dioxide gradually escape through tiny pores in the shell, and air enters the space they leave. As the air pocket at the blunt end grows, the egg becomes lighter without changing size. It may stand upright in water, or rise toward the surface when older. Floating does not always mean it is spoiled; it is more a sign of increased air inside.

Some people read the water test the other way round, assuming an egg that sinks is safe. But all the test tells you is the size of the air pocket, that is, how old the egg is. The USDA says even clean, uncracked fresh eggs may contain Salmonella. The bacteria can get in while the egg is forming inside the hen. That is why even fresh eggs are safest cooked until both the yolk and white are firm.

The water test is a household way of roughly judging an egg's age. Many households still fry or boil a floating egg if it has no smell when cracked. But if the shell is cracked or it smells strange, it is better to leave it unused even if it sank. Adding a lot of salt to the bowl can make a perfectly ordinary egg float, so the test should use plain water. In Korea, the first four digits on the shell tell you the day the egg was laid. This laying date has been printed since 2019, so 0823 means it was laid on August 23. Older eggs have their uses too. Over time the white becomes more alkaline and clings less to the shell membrane, so boiled eggs peel more easily.