Bruce was Nate's goldfish.
Nate got him on December 13, 2000 to try to prove to his father that he had the responsibility to take care of a dog. However, he dropped the bowl, shattering it. Bruce survived, and was put into another bowl.
Nate left the bowl next to an open window and the next day, Nate decided to show Bruce to Francis. By then, the water had frozen, and so had Bruce.