I googled it for you, and this is what i came up with
"You can tell if an egg is fresh by taking it for a swim. Carefully place eggs in a bowl full of water. Fresh eggs will sink down and lie on their sides at the bottom of the bowl. Semi-fresh eggs will stand on end and bob in the pool. Old eggs will float and should be discarded. Why? It all depends on the size of the air cell inside the shell of the egg. The older an egg is the larger that air pocket will be, giving the egg a built-in life jacket!"