You're an old-timer if you can remember when setting the world on fire was a figure of speech.

The joke hinges on a reversal: “setting the world on fire” used to be harmless metaphor for ambition or brilliance, but in an age of nuclear weaponry it can be heard as a literal description of catastrophe. Calling someone an “old-timer” if they remember the phrase as purely figurative suggests how quickly historical conditions can change the emotional charge of everyday language. The line is comic, but its comedy is edged with anxiety: progress has made the unimaginable imaginable, and even idioms—our most routine verbal habits—register that shift.