The key is an item that a villager uses to get into his or her house. Every so often in Animal Crossing: City Folk, a villager will be seen walking only around the outside of his or her house rather than the town. The villager then states that he or she is locked outside and has lost the house key somewhere around town. The villager may further reveal the location, which is the river - including the waterfall and river pool. The key can be found after the player has talked to the villager.
The key appears as a small fish shadow in the river, about the same size as a crucian carp. Once the player has fished it out the river and returned it to the villager, the player will receive a small reward, usually furniture, a gyroid, or a small amount of Bells.
If the player fails to find the key, the villager acts normal the next day, suggesting that he or she eventually found a way into the house.