Suppose you are explaining the theory of relativity to a friend, and you tell him that nothing can go faster than 300,000 km/s. He says that is obviously false: Suppose a spaceship traveling past you at 200,000 km/s, which is perfectly possible according to what you are saying, fires a torpedo straight ahead whose speed is 200,000 km/s relative to the spaceship, which is also perfectly possible; then, he says, the torpedo's speed is 400,000 km/s. How would you answer him?