ELI5 - How do eggs work?

Imagine a chicken egg as a tiny, round lunchbox with a house built into it.

The chicken’s body makes eggs even if there is no rooster around. This surprises many people. Most eggs we eat were never going to become chicks. They are just food. Inside the chicken, an egg starts with the yolk. The yolk is the yellow part. It is full of fat and nutrients. Think of it like a packed backpack full of energy.

As the yolk moves along inside the chicken, it gets wrapped in the egg white. The egg white is also called the albumen. This part is mostly water and protein. Its job is to protect the yolk and give extra food if a chick ever grows. Next, a thin skin forms around everything. Then the hard shell is added. The shell is made mostly of calcium. It is strong, but also a little bit porous. That means it has tiny holes you cannot see. Air can pass through those holes.

Now the egg is ready to come out. The chicken lays it. This does not hurt the chicken. It is something her body is built to do. If there is no rooster, the egg will never turn into a chick. It stays an egg forever. That is the kind you buy at the store and eat for breakfast. If there is a rooster, things change. The rooster can fertilize the egg before the shell forms. This means the egg has the instructions to grow into a baby chick.

If that fertilized egg is kept warm, usually under a mother hen, the chick starts to grow. The yolk becomes its food. The egg white gives water and protein. The shell keeps it safe from the outside world. As days pass, the chick grows bigger. It forms a beak, legs, and feathers. Near the end, it uses a small sharp bump on its beak to crack the shell. This is called pipping. Then the chick pushes and breaks out.

So an egg has two possible jobs. It can be food for people and animals. Or it can be a tiny growing room for a baby chicken. Either way, an egg is a clever little package. It is made to protect, to feed, and to work all on its own. That is why eggs are so simple on the outside, but so smart on the inside.