Animals

Kangaroos weigh less than 2 grams when they are born. The tiny baby, called a joey, climbs up its mother's belly and into her pouch. The mother can't touch it because it is so tiny. She licks a path in her fur for it to travel along. Inside the pouch it grabs onto one of four teats and remains attached to it for about nine months.
Milk is automatically fed to the joey, and the milk changes according to the joey's needs as it grows until it no longer needs milk.
At nine months the joey will start to leave the pouch for increasing periods of time,returning always to the same teat for a feed until it is weaned.
A female kangaroo generally has another baby in her womb 'in suspense', which means it has developed just a little bit and then has stopped and waited. When a joey leaves the pouch, the mother starts the development of the one in her womb again, and it is born a few weeks later. Then she will have one tiny helpless joey in her pouch, drinking the kind of milk it needs to develop, and she will have another joey that is out of the pouch but which returns to feed on milk from another teat in her pouch. That milk will be different from what the tiny joey is drinking, because the older joey needs milk that will help it get strong as it hops around.