a man is bored and decides to build a house in the woods. when he does so, he cuts down trees (natural habitats of animals), burns old things, and creates a lot of pollution. slowly the animals leave or die off due to his presence. when he finally is done with his house, he takes a look at what he has done, apologizes for his pollution and pain and suffering he inflicted on the surrounding habitat and dies of his own air pollution the next day (all very unlikely to happen, but this is an exaggeration). this is a negative externality because the producer/consumer (the man) is producing and consuming the construction of the house and reaping the benefits for his own, but letting negative effects fall on his surrounding habitat.
i am bored and sad and in need of a home animals
look i built this house
well im done with my house, but now i must burn all of my waste
i am sorry that i polluted and scared off your friends from land that was rightfully there
the man died the next day from all of the air pollution he created. the "rare" animals along with him; both will never come back:(