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