Say you've got two packages of hamburger buns. One of them is open and missing 4 buns, the other one has never been opened at all. Both of them have been sitting in the same spot, on top of the refrigerator. Why is it that the one that has been opened has not become moldy, and the one that has never been opened has?
ETA: The packages were bought on the same day and have the same expiration date