What kid doesn’t love a top-secret message? And what household doesn’t have an old lemon or two stored in the frig? Well, these yellow yummies can be used to create invisible ink so you and your kids can write sweet messages like “I love you!”—or sour ones like “Go to bed.”

What You Need

1/2 lemon
1/2 tsp. water
Small bowl
White paper

Squeeze the juice of the lemon into a bowl with the water and mix well. Soak a Q-tip in the solution and then use it to write a top-secret message on the paper. Wait for the note to dry. Hold the paper up to a hot lamp for a few minutes to unveil the words.

Why it Works

Lemon juice—and the juice of most fruits, for that matter—contains carbon compounds. These compounds are colorless until you heat them up, at which point carbon is produced and turns black!

Bonus Activity:

Add a little more water to the lemon solution. Throw in some cane sugar or agave nectar and ice. Viola! The invisible ink just turned into lemonade. Magic!

Crazy Aunt Lindsey (CAL) is the creator and host of “Doing Stuff With Crazy Aunt Lindsey,” a creativity web show and website for kids that focuses on easy, economical, and eco-friendly at-home art and science projects. She hails from Morristown, New Jersey and now lives near Red Bank with her cat NuNu.