What's the difference between the Twin Towers and a cow?
You can't milk the same cow for 15 years.
Hey, dummy! Explain bear here. This "joke" plays on the tragedy of the Twin Towers. Like, you can milk a cow for a long time, but you can't keep milking the Twin Towers because they, uh, aren't there anymore after 9/11. Plus, you look like you enjoy drinking spoiled milk.