Invisible ink is a great way for kids to have fun writing secret messages to each other and it can also demonstrate scientific principles. Make cabbage water by boiling purple cabbage in water until the water turns purple. As the cabbage water cools down, use cotton swabs to write a message on paper in lemon juice and let it dry. Brush cabbage water over the message with cotton balls. The acid in the lemon residue should react with the cabbage water to make the message stand out in red.
Spies have to be careful not to leave fingerprints behind when they sneak into forbidden areas. Kids can practice taking and identifying one another's fingerprints. Have each child color a dark area of pencil on a sheet of paper, then roll the pad of one finger in the pencil mark and place the darkened fingertip on the sticky side of a piece of tape. Kids can tape one another's fingerprints to a sheet of paper and compare them to find who has each of the three characteristic fingerprint shapes: whorl, loop and arch.
This simple chemical analysis visibly breaks ink down into its component parts. Give each child a different non-permanent marker and secretly choose one to write a ransom note. Cut the ransom note into 1/2-inch strips, then reassemble it. Give each child a strip of the original note and one strip of plain paper. Each child can choose a friend as a "suspect" and have the suspect use her marker to draw a circle on the plain strip. Place the ends of both strips into a cup of water with the marks above the water line and compare the resulting patterns to find whose pen wrote the note.
A different form of invisible ink reveals its message under heat instead of using a chemical reaction. Use cotton swabs to write a secret message on paper using lemon juice or milk. Let the message dry completely. If desired, you can stack a heavy book or two onto the paper to minimize wrinkles. Warm the note over a light bulb under adult supervision. The paper and the organic materials in milk or lemon juice should respond to the heat differently, causing the message to appear in dark letters.