The Joke: Why are shopping centers boring? Because if you've seen one, you've seen the mall. There is a phonetic similarity between the word "mall" and the word "all." The joke is a play on the phrase "If you've seen one, you've seen them all." * Means everything is the same and nothing is unique. The Pun: "Them all" sounds exactly like "the mall." It's funny because it uses a common phrase to complain that every shopping center looks exactly like the last one.