This simple experiment is great for young kids, but must be conducted with adult supervision because it involves using the kitchen stove.
Invisible ink has fascinated kids for generations. You might think that it would be difficult to make, but it's not.
All you'll need for this experiment is some milk; some white paper; a clean cotton swab or small, clean paintbrush; the kitchen stove; and help from an adult.
The most difficult part of this mini-project is choosing the message that you'll write with your invisible ink. Maybe you'll want to pretend that you're a spy on a secret mission, writing messages that would lead to big, big trouble if they were discovered. Choose your words carefully.
All you need to do is, using the brush or cotton swab, write your message with milk on the paper. Don't use a lot of milk and soak the paper. Use just enough to make a thin coating.
Once you've finished writing, ask the adult who's helping you to turn on the kitchen stove to a low heat. Very carefully, using kitchen mitts, hold the paper about a foot above the warm burner. If you don't see results, you may have to make the burner a little hotter.
Do not, however, hold the paper low over the burner. As the paper heats up, your message will -become brown and you'll be able to see what was written.
You want to know why? It's because chemical compounds in the milk have a low burning point. When the paper gets warm, those compounds will heat up and turn brown, while the paper will be unaffected.
You also can try this experiment with other liquids, such as orange juice, lemon juice, vinegar, and apple juice. Just be sure to follow all safety rules and have an adult around to help you.
Excerpted from The Complete Idiot's Guide to Science Fair Projects © 2003 by Nancy K. O'Leary and Susan Shelly. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.