Opossums enter attics for warmth and safety. Once an opossum picks your attic as its home, it will stay there as long as it can. Opossums enter by climbing trees and gutters to get to the roof and then taking advantage of weak points in the roof, soffit, or eaves. In the attic, the opossum will nest and nurture a litter. They create problems by dislodging sensitive attic structures, entering air conditioning vents, keeping the homeowners awake, and leaving a large mess of droppings / feces.
The opossum is the only marsupial in North America. This means the animal carries its young in a pouch, much the same as does the Australian kangaroo. Once a female opossum mates, she gives birth a mere 13 days later to a litter of roughly a dozen baby opossums that are each no bigger than a honeybee. These tiny, blind, and naked babies crawl on their own all the way to their mother's pouch. They each latch on to a teet from which they receive milk. They remain in the pouch for nearly three months. Once the young opossums leave the pouch, they're still not ready to face the world on their own. For the next 10 to 15 days they go about clinging to mother's fur. Eventually they become too heavy to hang on during these trips and one by one fall off. By the time this happens, the young opossum is fully weaned and able to forage for himself.
The opossum is perhaps best known for faking death ("playing possum") as a means of defense when attacked. While he is capable of falling over on his side, his mouth open in a death-like grin with saliva running out, from which state he cannot be roused until the danger has passed, this is usually done only as a last resort.
A threatened opossum will most likely look for the nearest exit and run away (or more accurately "waddle away," since they cannot move particularly fast). They will sometimes bare all 50 of their teeth, hiss, or even growl. With such displays they appear quite fierce, but actually the opossums is not an accomplished fighter and is rarely aggressive.