As a kid, I loved codes, "secret" passageways, and optical illusions. Anything that seems to happen mysteriously is a fun experience for kids and grown-ups alike. When you're in the mood for a little seemingly unexplainable fun (or you're learning about chemical reactions), this secret message activity is the perfect solution.
Students start with a blank piece of paper, some q-tips and some acidic liquids. In my class we tried out milk and lemon juice. Using a q-tip as a brush and the liquids as invisible ink, students can draw or write any secret picture or message they'd like.
Once the painting is done, the next step is to reveal the hidden message. We started with the iron on very low, but then we ended up turning it up a little bit to make the message appear darker. The heat from the iron will burn up the acidic liquids, turning the paper dark in places that were painted.
We found that the lemon juice appeared much more clearly than the milk. It would be fun to test and compare the results of using several different "paints." This activity would also work in a social studies lesson on espionage.