A joey is a baby marsupial, especially a baby kangaroo. Marsupials are mammals that carry their tiny, underdeveloped babies in a pouch on the mother's belly. When a kangaroo is born, it's only about the size of a lima bean and can't see, hear, or even regulate its own body temperature. The newborn joey crawls through its mother's fur into her pouch, where it stays safe and warm for months while it grows.

Inside the pouch, the joey drinks its mother's milk and gradually develops until it's strong enough to peek out and explore. By the time a kangaroo joey is several months old, it might hop out of the pouch for short adventures but quickly jump back in if danger appears. Eventually, the joey becomes too large to fit and must live on its own.

The term joey applies to other marsupials too: koalas, wallabies, wombats, and some possums all have joeys.