Gases build up inside the shell making it more buoyant as eggs age.
This is the best way to test whether an egg has gone rotten without having to break open the shell,
risking the foul odor escaping.
When an egg is extremely fresh it will lie on its side
at the bottom of a glass of water. As it ages,
the egg will begin to point upwards,
and will finally float completely when it has gone bad.
Fresh eggs have a very firm white,
whilst old eggs have a very watery white.
This is why it is best to use the freshest eggs
possible for poaching and frying.
Older eggs are perfectly good for omelets or scrambling.