1. and no one is around - Keep It.
2. and you saw the person right in front of you drop it - Give It Back.
3. and you saw the person 100 steps in front of you drop it - Keep It.
4. and you saw the person 100 steps in front drop it and no one else is around - Give It Back.
5. and you saw the person in front of you drop it and then they turn and go into a building - Keep It.
6. and the person who dropped it had just insulted you - Grin, then Keep It.
7. and the person who dropped it owes you twenty dollars - Keep It, then wait and see if they ever try to pay you back. If they do, say, "oh, that little twenty dollars you owed me? oh, don't worry about that! What are friends for?"