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 juice on a letter.Three Lions/Getty Images Humans have had a long ...

Which invisible ink pens are best? Spies and conspirators have used invisible inks for millennia to send messages. In the modern age, invisible ink pens are an excellent way for grown-ups to ...

Coded messages in invisible ink sound like something only found in espionage books, but in real life, they can have important security purposes. Yet, they can be cracked if their encryption is ...

Which invisible ink pens are best? Spies and conspirators have used invisible inks for millennia to send messages. In the modern age, invisible ink pens are an excellent way for grown-ups to ...