Answer: I think it is Nancy is in the library looking for a book. She is talking to herself and consequently disturbs a few people in the library.
Explanation:
There are good externalities and bad externalities in economics. A good externality is when for example a farm of flowers is situated next to a producer of honey. The farmer benefits from the polinization of his flowers and the honey producer from the pollen and nectar that bees take from the flowers to produce honey. A negative externality is something like in the case mentioned of Nancy who reads aloud and disturbs other people in the library and doesn't let them read peacefully.