E (age 5), and I were inspired by Jim Wiese's Spy Science: 40 Super Sleuthing, Code-Cracking Activities for Kids, to try our hand at some undercover art, today.
First, I gave her a piece of paper, a bowl of milk, and a q-tip.
She dipped the q-tip into the milk, and used it like a paint brush, to draw a picture of a weather girl (I think). Not wanting to be left out, I wrote a quick message on another sheet of paper, myself, and we left them to dry for a couple of hours.
When we returned to our papers, the milk had dried clear, if a little shiny. I thought my message was still too legible, so I crinkled my paper up, hoping it wouldn't ruin the "ink".
Then, we covered the papers with bits of graphite, by rubbing a pencil, on sandpaper, over the sheets.
We smoothed the graphite out with a napkin, and our drawings, magically, appeared.
Wiese suggests using whole milk, because it's the fat, the graphite sticks to. So, the higher the fat content, the better the experiment will work. We were happy with the success we had with 2%, though.
It even worked, for the most part, on my crumpled up paper, where the message had been rendered completely invisible.
It's great to be a homeschooler.