Have you ever bought something that had a negative effect that you didn't expect? You could call that a negative externality, I like to focus on how the beginning of the word sounds like extra. If you buy a TV and you watch it all the time, a negative externality could be the damage to your eyes or that you become obese because of it. Everything has a externality, just think about whether the positive externality is worth tolerating the negative, you give some and you loss some.