The idea of invisible ink had been around for a long time. Write using lemon juice as ink, for instance, and the writing will be invisible. Heat the paper, and the writing will appear, looking as if it were written in brown ink. British agents used two types of ink. One could be made visible by holding the paper over the flame of a candle; the other could be read by applying a common chemical. Major John André, the chief British intelligence officer in New York, told his agents to put an F in the corner of letters containing invisible ink needing fire to read and A for those needing acid. —George Washington, Spymaster, Thomas B. Allen What is the central idea of the passage?

Question

The idea of invisible ink had been around for a long time. Write using lemon juice as ink, for instance, and the writing will be invisible. Heat the paper, and the writing will appear, looking as if it were written in brown ink. British agents used two types of ink. One could be made visible by holding the paper over the flame of a candle; the other could be read by applying a common chemical. Major John André, the chief British intelligence officer in New York, told his agents to put an F in the corner of letters containing invisible ink needing fire to read and A for those needing acid.  —George Washington, Spymaster, Thomas B. Allen  What is the central idea of the passage?

GPT-4o mini · Answer

The central idea of the passage is to explain the concept and use of invisible ink in espionage, particularly during the British intelligence efforts in the American Revolutionary War. It highlights how different types of invisible ink were utilized for secret communication and the methodical approach taken by British agents, specifically mentioning Major John André's system for indicating which method was needed to reveal the hidden messages.