Either squeeze a lemon or be lazy like me and use that store bought lemon juice that comes in the little plastic lemon! Put the juice in a small dish and use a cotton swab to write your secret message.
BAKING SODA & WATER:
Mix together equal parts baking soda and water in a small bowl. Again, use a cotton swab to create your message.
Once the messages are completely dry and can no longer be seen on the paper, hold the paper over a heat source and the message will magically be revealed to guaranteed “oohs and aahs!” (Obviously an adult should complete this step, especially with young children, so as to avoid burns and flaming paper.)
By the way, we found that the baking soda produced a slightly darker result than the lemon juice.
For those whose children channel James Bond rather than Martha Stewart, a high-tech invisible ink spy pen complete with built-in ultraviolet decoding light might be just the ticket. My son found this one in his Christmas stocking:
LINKS – More about invisible ink:
The Naked Scientist – Secret Messages-What Makes an Invisible Ink?
Kidzworld-How Invisible Ink Works
Science Project Ideas – Invisible Ink (this site has some other interesting methods too)