Pick a product or behavior that has a large externality effect tied to it. Remember, there must be a third party (or parties) who is affected. Don't use garbage, pollution, or fishing stocks in your example since I've already covered that a lot.

Describe the externality and why it counts as such. Say who is/are the third party who is not part of the market. State whether it is a positive or negative externality. You can use personal knowledge, google reputable information and provide a link, and so forth. If there's a group of people the cost falls on, who are they? Why them? Are they compensated in any way, and if so, how?