The female butterfly lays a small egg on the underside of the leaf. After a few weeks later the egg hatches and out comes a caterpillar. A caterpillar eats its shell for nutrients it also eats milkweed leaves. Caterpillar is are eating machines. It goes on a diet so it eats milkweed. When it gets big enough it forms a pupa. After a few weeks the pupa it turns blackish then clear. Soon the pupa breaks open and out comes a butterfly. Itss wings are wet and they can't fly straight away. So they wait later its wings are fully ready.