Invisible ink was known as "sympathetic stain" during the Revolutionary War. By any name, it was an effective spying tool.

Add Yahoo as a preferred source to see more of our stories on Google. William Abel, a mail censor at Iowa's State Prison, Fort Madison, using a hot iron to bring out a secret message written in lemon ...

Prisoners, Lovers, & Spies is the first history of invisible ink and secret communications revealed through thrilling stories about scoundrels and heroes and their ingenious methods for concealing ...

Invisible Ink is one of the standout features Apple introduced to iMessage back in 2016, part of a suite of message effects designed to make texting feel more expressive. While others like "Slam" and ...