Block of houses, all appearing identical. By each doorknob a light, red or green. Someone comes around the corner, goes to the first door with a green light, inserts his key, opens the door. He is home. What is happening, of course, is that his key contains the complete description of the inside of his house when he left it that morning (or last week or last month or ...). The houses are all identical until a key goes in (red lights are occupied houses). Now we add one crazy person who actually thinks coming home to the same house matters: Duncan