I Spy…Fun! A Classic Invisible Ink Recipe for Little Amateur Agents

Photo by Ronnie Comeau / Stocksy United / 745921

This weekend, your mission is CLASSIFIED––should you choose to accept it, your family-fun objective is to whip up a batch of invisible ink and write thrillingly cryptic messages and spine-tingling secret codes to each other! Spies have used invisible ink to pass along covert communications for centuries—and your littles will love using a secret technique of their own.

Our version of invisible ink was inspired by a recipe from Spy School, just one of our awesome summer school-age programs (we offer all sorts of summer adventures: Take a look!). The recipe calls for lemon juice and water, but you can actually make invisible ink from all sorts of ingredients, including milk, orange juice, baking soda, sugar water, vinegar, and soapy water. Consider trying a few different versions of invisible ink with your amateur agent.

Pro-spy tip:  Donning a fedora, sunglasses, trench coat, or mustache will help disguise you both from enemy agents—and make the whole afternoon even more fun.

Materials

  • Cotton swabs
  • Lemons
  • Water
  • Cutting board and knife
  • Juicer
  • Measuring cup
  • Mixing bowl and spoon
  • Plastic eyedropper (or a small spoon)
  • White paper
  • Lamp (with at least a 60, 75, or 100 watt bulb)

Process

  1. Cut the lemons in half. Let your child try using the juicer to squeeze lemon juice into a small bowl—offer assistance if needed.
  2. Have her use the eyedropper (or small spoon) to add a few drops of water to the juice. Stir it all together and presto! Your invisible ink is ready for code writing.
  3. Dip a cotton swab in the mixture and use it like a pencil to write words or draw pictures on white paper. (Pro-sleuthing tip: Use a light touch when applying your ink for a faster drying time and a more readable final product.) Try writing spy-like messages to each other: “Meet me outside at noon,” “The dog barks at midnight,” or “I’ll be the girl in the red hat.”
  4. Set the paper aside to dry for at least 25 minutes—it’s ready when all the writing has disappeared. 
  5. When the paper is dry, turn on the lamp and help your child hold her piece of paper near the light bulb (make sure she doesn’t actually touch the hot bulb). The message on the paper should appear in pale, toasty brown lettering. Why it works: Lemon juice is mildly acidic, which weakens the paper wherever it is applied. When the paper is held close to a heat source, the weak parts of the paper begin to “burn” (turn brown) before the untouched portions, revealing the hidden message.

Have a crew of kiddos? Let them try passing messages to each other, scripting secret directions to uncover a hidden object, or writing steps to accomplish a covert task (i.e., you distract Mom, I’ll grab the cookies).

To keep leftover ink for more mysterious machinations, put it in a sealable container and store in the fridge for up to three days.

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