If you like this post, you should check out our cooking with kids series, our favorite nature books for kids, and our favorite kids’ camping gear.

Fun back story: the idea for today’s post was hatched during a trip to our favorite thrift store, which is slowly becoming “the infamous trip to the thrift store” because so many awesome items were purchased.

We stopped in on a whim to pick up some thermoses, but left with so much more, including a pair of spy glasses for Alex. One thing led to another, and he wanted to write notes in invisible ink. Snow was predicted for a couple days later, so I made sure that we had the supplies on hand, and this little activity made for a fun morning indoors!

You might be surprised to learn that you probably have everything you need to start writing secret spy notes tonight…

We got the instructions for this project from one of my favorite kid-centric reference book: The Dangerous Book for Boys. Rather than always turning to the internet and screens for information, I love knowing that I can send the boys to grab this book from the shelf and it often has the answers!

Materials

• lemon juice and/or milk
• paper (we liked the quality of thicker art paper)
• q-tips for writing
• candle for revealing the message

How-to

• using the q-tips, write a secret message on a piece of paper
• you could also write a real message in ink on a different part of the paper our source tells us that’s a great way to hide the message from other spies
• once the invisible ink has dried, you can reveal the secret message by holding the paper over a candle with the message side toward the flame
• it may take a little bit of time, but eventually your secret message will be magically revealed!

What’s going on?

The sugars in the lemon juice and milk start to burn from the heat of the flame. That burning reveals the secret message by turning the area with invisible ink brown.

Lemon juice and milk are the two liquids called for in this activity because they’re common in most households and they dry relatively clear, making for an exciting magical reveal when they burn.

Tips and Tricks

In our (amateur) experiments we found that the milk produced a more pronounced final message than the lemon juice. The message saying the “ghoul has secured an identity” was done with lemon juice, while the one saying “the eagle has landed” was done with milk.

It also helped to be liberal with your application. The milk message above was created by the heavy handed application of kiddo while the lemon juice message was applied a little by more conservatively.

Overall, we’d give this activity a solid 10/10. It was super easy to do with household materials and almost no cleanup. And the wow factor made it fun for both kids and adults.

So, if you’re looking for holiday gifts or just some fun on your next snow day, get the spy glasses and put together a little invisible ink kit for your favorite spies.