...the mundane egg, which is but an elongated circle, contains everything in itself and is the true microcosm.-- Matilda Joslyn Gage, Woman, Church and State

A microcosm of yummy, protein rich, ricket-preventing fun that is! Eggs contain valuable amino acids and vitamins, and are regarded as an excellent source of protein by vegetarians, struggling artists, and the US Food and Drug Administration.

Rotten eggs, on the other hand are just unspeakably awful. If you have ever had the misfortune to break one and get it all over you, you will know why last one in is a rotten egg! is such a dreadful thing to say.

Fortunately, a simple test exists to determine if an egg is as edible as it looks. There is a small air pocket in the large end of an egg. When the egg is fresh, the pocket is only about 1/8th of an inch deep and about as large around as a dime. As the egg ages, it loses both moisture and carbon dioxide so that the size of the air space increases, making the egg (among other things) more buoyant. So, if you submerge a very fresh egg in water, it will lie on the bottom. An egg that is a week or so old will lie on the bottom but bob slightly. An egg that is three weeks or so old will balance on its small end. And bad egg will float.

Here's a whimsical poem by Scott Mathews on the subject of this method of egg testing!
Can you eat that egg?

If not sure you ought-ter,
then place it in water.
If it lies on its side,
then it's fresh; eat with pride.

After three or four days,
at an angle it lays.
But, it still is a treat,
so go on and eat.

Ten days, stands on end,
in your baking 'twill blend.
'Cause it's definitely edible,
in your baking, incredible.

But, if it floats on the surface,
that egg serves no purpose.
'Cause a floater's a stinker!
Out the back door best fling 'er!