system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The process of animals receiving oxygen inside eggs, and how they transition to breathing once hatched.

How do animals receive oxygen inside eggs?  Then once hatched, how do they know to start breathing and or how does the process instantly change?
gpt
I read about this at a dinosaur exhibit. When bird or reptile lays an egg, two membranes are formed below the egg shell. Imagine two layers between egg shell and the egg white. And between two layers is air sack which holds oxygen for the developing embryo. But this air sack needs to be replenished and the embryo must be able to release carbon dioxide. And here is where the amazing eggs shells come into play because they are made of tiny pores. A chicken egg is said to have about 7000 pores, approximately. As the chick grows, the pores on the egg shells allows oxygen and carbon dioxide to circulate to and from the air sack. 
Once the chick is fully formed, it slowly positions it's beak under the inner layer and pierces through the inner layer to breath from air sack. And then it pierces through the shells and starts breathing oxygen from the atmosphere.