How we did it:
- lemon juice
Gather up your materials. You can get your lemon juice from a fresh lemon or from a bottle, and any kind of salt will work just fine. Also, if you don't have a paint brush, you can use a Q-Tip.
If you are using a real lemon, then an adult (or older child) should cut the lemon in half and squeeze the juice into a small dish for painting.
Then let the kids paint away with the lemon juice on to some paper. I had my daughter draw a picture of an apple and I helped her paint an A on the paper. It was her "secret message."
Before the lemon juice dries ,sprinkle salt onto the paper and let it sit for a bit. My son loved playing with the salt... and tasting the tangy salt when it mixed with the lemon juice. (While this mixture wouldn't hurt, I wouldn't recommend eating the salt.) Brush the salt off and let the paper dry.
The paper dries clear and white. To see what was painted on, simply take a crayon and rub gently over the paper. The area that was painted shows up darker than the part that was not painted. The kids were so surprised to see the secret messages. They excitedly started drawing out secret messages to send off to their grandparents!