Testing whether an egg floats or sinks in water is a quick, easy way to tell how fresh the egg is. Fresh eggs sink completely and come to rest horizontally at the bottom of a container filled with water.
Eggs that are still edible but may be on their last leg stand upright in water, but still have contact with the bottom. Eggs that have gone bad and should not be used float freely away from the bottom of the container. As eggs age, the liquid inside evaporates, leaving extra air inside of the shell. This air causes the egg to float.