Ignore the ‘fresh’ date? How2 determine the freshness of an egg.

Old eggs float in fresh cold water because of a large air cell that forms as the egg cools after being laid. As the egg ages, air enters the egg and the air cell becomes larger and this acts as a buoyancy aid. Generally, fresh eggs will lie on the bottom of the bowl of water.

How does a baby bird breathe inside the egg?

The two membranes pull apart a little and create a small pocket or sack of air. As the developing bird grows, it breathes in oxygen from the air sack and exhales carbon dioxide. Several thousand microscopic pores all over the surface of the egg allow the CO2 to escape and fresh air to get in.

The oldest and easiest way to tell if an egg is fertilized is called candling the egg. It is literally holding the egg up to a lit candle {not to warm it, but in order to see inside of the egg}.

https://mariherigstad.wordpress.com/2016/01/26/how-do-eggs-breathe/