Why is a possum a marsupial?

Answer 1

It has a pouch.
However, a pouch is not what makes a possum a marsupial. Not all marsupials have pouches, e.g. the numbat has a mere flap of skin, but in animals where the pouch is absent, the young are still born undeveloped, and they cling by instinct to the underside of their mother's belly, still firmly attached to teats which swell in their mouths.

Marsupials are mammals which usually have pouches in which they rear their young. However, of more importance is the fact that marsupial young are characterised by being extremely small and undeveloped at birth. At birth, they take a long, arduous journey from the birth canal, driven purely by instinct, grabbing hold of the mother marsupial's fur which she has cleaned and made easier to traverse with saliva, to reach the pouch. Upon reaching the pouch, they latch onto a teat which swells in their mouth to prevent them from being accidentally dislodged during the mother's movements. There they stay for months, to complete their development.

Possums also share other marsupialian characteristics. Female marsupials have two vaginas, or what are called paired lateral vaginae. These are for the purpose of transporting the sperm to the womb, but there is a midline pseudovaginal canal for actually giving birth. As well as two vaginas and two uteruses, female marsupials have two fallopian tubes and two cervixes. Most male marsupials, with the exception of the largest species, the Red Kangaroo, Eastern Grey and Western Grey Kangaroos, have a "bifurcated" or two-pronged penis to accommodate the females' two vaginas.

Answer 2

so they can stay warm in the winter