A cool project for kids of all ages

If your kids are like mine, and I suspect that they are, they love anything that has to do with being a spy. Secret codes, special listening devices, spy goggles.

What about invisible ink?

This is a really simple one.

Squeeze some lemons or limes into a glass. This juice will be the ink.

Give them a small paintbrush or a Q-tip to use as writing implement.

Have them write the message on a sheet of white paper. When the “ink” dries it will be invisible.

To make the message reappear, hold the sheet of paper over a heat source. A hot water bottle, heating pad, in front of your wood burning stove, a heat register are some ideas.

When I originally read about this disappearing invisible ink several years ago, it suggested holding the paper over a candle. That just requires a level of supervision that I am not sure young spies will appreciate. And frankly frightens me that one of my spies might try and do it on their own in a corner or closet somewhere.

It might be fun to write secret messages to the children and then have them figure out how to make the message appear.

And for the science behind this:

Lemon juice and lime juice contain carbon. The carbon darkens when put neat a heat source, allowing you to read the message.

15 Responses to “A cool project for kids of all ages”

  1. Katie Says:

    You can also just leave it in a warm sunny spot for a while.

  2. Brigitte Says:

    Ah, I’ve always been leery of the whole candle thing, I didn’t know it would work with milder heat sources - thanks!

  3. tanya Says:

    We just put it in the oven on low.

  4. Montserrat Says:

    My daughter did this last week and mailed it to a friend. They thought it was one of the coolest things, “Like what they did on National treasure!”

  5. marta Says:

    Ever since I read about it in the Secret Seven (or was it in the Mystery?) books series by Enid Blyton when I was about 8 or 9 I’ve done the invisible ink on and off, with my brothers, later with younger cousins and now am willing to do it with my own kids (the oldest being almost 7 and ready for anything magical/scientific).
    Thanks for reminding us!

    Marta from Lisbon

  6. Brigitte Says:

    Marta from Lisbon, CT? If so, we are practically neighbors, cool.

  7. marta Says:

    Oh, no :(
    Lisbon, Portugal!
    Didn’t know there was a Lisbon in Connecticut, though ;)

  8. Holly Smith Says:

    Cool! We’re trying it! Thanks for sharing…

  9. Christine Says:

    I’m going to try a blowdryer. We used to hold it next to a lightbulb, but now we only use compact flourescent - good for the earth - bad for secret spies!

  10. maggie Says:

    Cool - I can’t wait for my little one to be bigger - she’ll love this! (Or I will?)

  11. Mel Says:

    What a great idea!
    Will have to try that one.

    A good way of getting more juice from the lemon is to roll them for 30 secs or so on the table. Another way I’ve found is to heat on high for 20 secs in the microwave. The lemon yields a lot more. Then of course you have to use the peel in a cake :)

  12. Much More Than A Mom Says:

    Thanks for the great idea of a safer heat source. I’ve always wanted to do this with my grade one students and now I can!


  13. Christine Says:

    My kids are so funny, “Mommy, what is the ‘big family’ doing this week?” They like to check out your ideas, and what your kids like.

    Alright … the blowdryer didn’t work (it got overheated). I tried the open flame … caught the paper on fire … more than once.

    I’ll go with the leaving-it-out-in-the-sun next time. We gave up. It was getting too stinky … and … um … dangerous?

  14. greatexpectations Says:

    I’m linking to you, again, because you always have great activities for kids … Thanks

  15. Charisa Says:

    We once did this project at a kids daycamp. We used an iron to make the ink appear. At daycamp we had the leaders do the ironing… :)

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