Spies have been known to use invisible ink. This is ink that doesn't show up until something is done to the message.  For example, sometimes special chemicals must be poured on the document to reveal the message. Other times heat may need to be applied to the message. Sometimes these messages are written on paper, but some spies have been known to write messages right on their bodies with invisible ink, so they don't get caught carrying paper messages.

The actual science of writing hidden messages is called steganography. Sometimes the steganographic message may be written in invisible ink and sometimes the message appears to be something different.  For example, it may look like a shopping list, a picture, or some other cover message.  First, the steganographic message is encrypted. Encrypting the message is when it is put in a form that isn't understandable to anyone but the person who has the key to decipher it.  Next, a covertext is changed in some way, so that it contains the encrypted message.  For example, letter spacing or letter size might be changed to carry the secret message. Only the person who knows what technique was used will be able to understand the true message.

Here are two recipes to help you make your own invisible ink!

Lemon Juice Recipe

You will need:
Lemon juice
A small glass
Toothpicks
Paper
Iron

To make invisible ink, follow the directions below:
1. Pour lemon juice in your glass.
2. Soften the point of the toothpick in the juice.
3. Write your message in juice.
4. Get an adult to iron your paper. 

Your message should appear in brown.

Iodine & Cornstarch Invisible Ink Recipe

You will need:
2 tablespoons cornstarch
4 teaspoons of water
Hot plate or stove
Toothpicks
Pan
Iodine solution (1 teaspoon of iodine in 10 teaspoons of water)
Paper
Water
Small sponge

To make invisible ink, follow the directions below:
1. Using the pan, mix the water and cornstarch until smooth.
2. Ask an adult to heat the mixture for several minutes. Stir while it's heating.
3. Dip a toothpick into the mixture, and use it to write a message on paper. Let the paper dry.
4. After it has dried, read the message by dipping a small sponge into the iodine solution and wiping it over the paper. Don't get the paper too wet.

The message should appear purple.

Citations

Books

|Code and Cipher Basics| |Spies| |Bugs, Taps and Surveillance| |The Enigma Machine| |Invisible Ink| |Morse Code Cipher| |Picture Cipher| |Transposition Ciphers| |Pig Pen Cipher| |Hand Signal Code| |American Sign Language Code| |Jefferson's Wheel Cipher| |Substitute Cipher| |Alberti Cipher Wheels| |The Scytale Cipher| |Grid Cipher|