Secret Spy Notes: How Lemon Juice Becomes Invisible Ink
Did you know a lemon can help you write a secret message that no one else can read? Long, long ago, during a time called the Cold War, some people used tricks just like this to send hidden notes!
What You'll Learn
- Learn how lemon juice can become invisible ink - Learn what warming the paper does to hidden lemon-juice writing - Learn what a spy is and why some used secret codes long ago - Try writing your very own secret message
What Is Invisible Ink?
Invisible ink is a special liquid you can write with that dries clear, like it disappeared! Lemon juice is one kind of invisible ink. When you dip a cotton swab or a toothpick into lemon juice and write on paper, the juice dries and you can't see it anymore. But the message is still there, hiding!
Why Did Spies Need Secret Messages?
A long time ago, from about 1947 to 1991, there was a time called the Cold War. Two big groups of countries did not trust each other, so people called spies tried to learn secrets and pass along messages without anyone else reading them. Spies used tricks like invisible ink so that if someone else found the paper, all they would see was a plain, blank page!
Lemon juice has tiny bits of carbon hiding inside it. When you gently warm the paper — like holding it near a warm lightbulb with a grown-up's help — the carbon turns brown before the rest of the paper does. That's how the hidden words suddenly appear!
What happens to lemon juice writing after it dries on paper?
Why did Cold War spies use invisible ink instead of just writing a normal note?
Make Your Own Secret Message
Ask a grown-up to help you squeeze a little lemon juice into a cup. Dip a cotton swab in the juice and write a short secret word on white paper. Let it dry completely until you can't see it. Then ask a grown-up to help you gently warm the paper near a lightbulb (never a flame) until your secret word appears in brown. Show your finished secret message to a family member.
Flashcards — click each card to reveal the answer
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