Science Fun For Kids: Cranberry Spy Juice

Teach your child how to use cranberries to write a secret message and discover the science behind the experiment.

By Liz Heinecke
Nov 17, 2017



Science Fun For Kids: Cranberry Spy Juice

Nov 17, 2017

Your child will have a blast drawing pictures and writing secret messages using invisible ink made from baking soda and water. Once dry, the invisible messages can be revealed using the juice from cranberries.

What You'll Need

  • Blender
  • Colander or cheesecloth
  • 2 cups of cranberries
  • Water
  • Baking soda
  • Printer paper
  • Small paintbrush
  • Cotton swabs, like Q-tips
  • Blow-dryer (optional)

Safety Tips and Hints

When playing with cranberry juice, wearing an apron or old clothes is a good idea because it stains! Adults should always supervise children using the blender.

What to Do

Step 1: Ask your child to drop a cranberry into a cup of water and observe it. Does it sink or float?

Step 2: Help your child cut the cranberry in half. Ask him to find the air pockets that make it float.

Step 3: Let your young scientist measure 1 cup of water and 2 cups of cranberries into a blender.  

Step 4: Help your child blend the cranberries and water until it becomes a smooth and even mixture.

Step 5: Strain the cranberries using the colander or cloth and collect the juice.

Step 6: Let your child make invisible ink by measuring and mixing ½ cup water with 2 tsp. baking soda in a small cup or bowl. Don’t worry if the baking soda doesn’t dissolve completely.

Step 7: Let your child write or draw on the paper using a cotton swab that has been dipped into the invisible ink solution.

Step 8: Wait for the pictures and messages to air dry, or speed things up by helping your child to dry off the paper using a blow dryer.

Step 9: To reveal the invisible ink, have your child paint cranberry juice onto the paper where the image or message was created.

The Science Behind the Fun

Molecules called flavenoids give cranberries their bright red color. Cranberry juice is normally red or pink, but when you add it to a chemical like baking soda, it changes color to blue or green.  

This explains why your baking soda invisible ink turns blue when it touches cranberry juice, while the rest of the paper stays pink.  

Another interesting fact about flavonoids is that they are important for plants because, in nature, their bright colors attract birds and other animals to fruit. Animals eat the berries and spread plant seeds from one place to another so new plants can grow.

You can find more experiments like this one at, and in my books Kitchen Science Lab for Kids (Quarry Books) and Outdoor Science Lab for Kids (Quarry Books).
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