Operation Espionage

Today, your mission is to write with invisible ink and figure out the best way to reveal a secret message.

  • Target Age: 8-10
  • Skills/Subjects: Science
  • Related Episode: Episode 305: Mission Improbable


1. Get what you need:

  • 1 tsp. of baking soda
  • Water
  • 3 bowls, filled half way with grape juice, grape juice concentrate, and cranberry juice, each labeled
  • 1 bowl filled half way with water and a drop of red and blue food coloring, labeled "colored water"
  • 1 spoon
  • Cotton swabs
  • Cotton balls
  • Cup
  • Paper
  • Pencil
  • Paper towels

2. Mix the ingredients:

  • Combine 1 tsp. of baking soda and 1 tsp. of water in a cup and stir. You've just made invisible ink!

3. Write with invisible ink:

  • Fold a sheet of paper into four equal parts. Then unfold.
  • Dip a cotton swab into the mixture and use it like a pen to write a secret message on each of the four sections. Write something short (like your initials or "RUFF"), since there's not much room.
  • Let the paper dry completely, which takes about 10 minutes. You may need to fan it in the air to help it along. What happens to your messages as they dry?

4. Reveal the secret message:

  • While your paper dries, line up the different kinds of bowls filled with liquids.
  • On each section of paper, you're going to test a different liquid. Starting with one section, dip your cotton ball in one of the juices. Squeeze out the excess juice. Gently pat (don't rub) the cotton over one of the sections of paper.
  • Did your message appear?
  • Test each liquid on the three remaining sections of your paper. Use a new cotton ball each time. Does one liquid work better than the others to reveal the hidden message? What color is the message?


When you patted the juice on the dried baking soda, you caused a chemical reaction! Baking soda is what chemists call a base. The juices are acids. When a base and an acid come in contact, a chemical reaction can occur. Some chemical reactions result in a change in color, which is what happened here. The chemical reaction turned the invisible message bluish-green, and that's why you could read it!

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