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?
* Invisible ink isn't really invisible at all.
Lemon juice is the earliest form of invisible ink.
British agents used invisible ink to send secret
messages.

Read the excerpt from "home front diary." jimmy simmons stopped by today to tell me that he had enlisted in the army. he asked me if i would write to him, and i said i would. i told him that i've been writing to my uncle arthur and susan's cousin john for about a year, and as an experienced correspondent i had some advice about all the censorship. "don't write down any information about your location or the number of soldiers in your unit because the censors will cut all of that away," i said. "in my experience, loose lips not only sink ships, they make letters look like snowflake decorations! ” well, he belly laughed over that one. how is the author's perspective different from jimmy's? the author has been to war and she can give jimmy advice. jimmy has become a soldier and does not believe the author’s advice. the author has experience writing to soldiers but jimmy does not. jimmy has more experience at letter writing than the author.