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Koalas are marsupials, related to kangaroos. Most marsupials have pouches where the tiny newborns develop.
The joey feeds on this in addition to the milk it's already getting. Pap comes from the mother's intestines and contains bacteria that the joey needs to have in its own intestines so that it can digest an adult diet of eucalyptus leaves.
At about six months, the joey leaves the pouch to eat leaves but it needs nursing till 1 year.