Good and Evil

My friend is in town, staying with me, and she told me that when she came home the other night and rode in the elevator up to my apartment, there was a young boy and an old man in the elevator too. The young boy got out on another floor, and when the elevator doors closed the old man turned to my friend and conspiratorially said, "That boy is a twin. I can't tell the two of them apart. All I know is that one of them is good and the other one is bad."