Would you believe that you were once in an egg? During the earliest stage of development, animals grow inside an egg. This is true for insects, fish, birds, and for you. That’s right, even mammals (including people) have eggs. Nine months prior to our births, we all started out in a special type of egg that doesn’t have a shell.

Even though people don’t develop inside of an eggshell, many eggs do have shells. This helps keep the growing young (which are called “embryos” when they’re in eggs) protected from the world outside, while also making room for a bunch of food inside, kind of like the house you live in.

The walls help protect embryos from the germs, predators, and temperature outside, and make some space inside to keep extra food. Eggs are well stocked with food because moms typically put yolk in their eggs or they send food into the eggs during development.