Spies that use paper to send secret messages have two choices for hiding that message---secret codes or invisible writing. Interested parties can often recognise the obvious structure of secret codes. The invisible writing method, on the other hand, conceals the real message under a false one. Lemon juice is the most easily available "invisible ink." The acidic nature of the lemon destabilises the paper and the juice leaves behind its sugars so that later, when someone exposes the paper to heat or red cabbage water, the sugars in the weakened paper change colour, and the message materialises.
- Skill level:
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Things you need
- Matches sticks, toothpicks or cotton swabs
- Small dish or paper cup
- White paper
- Lamp with a 100-watt light bulb, candle, iron or blow-dryer
- Cabbage water
- Spray mister
Squeeze the juice of one lemon into a paper cup or small bowl. Set out one piece of white paper for writing the secret message on.
Dip one end of a writing implement such as a matchstick, toothpick or cotton swab, into the lemon juice and write the secret message on the paper. Set the paper aside, allowing the "ink" to dry.
Hold the paper with the secret message close to a heat source, but not so close that the paper will catch on fire; use a lamp with a 100-watt bulb, a heated iron or a blow dryer. As the page heats up, the sugars from the lemon juice will turn brown and reveal the message.
Slice up some red cabbage, place it in a pan of water and boil it until the water turns a dark purple to reveal the message with cabbage water. Remove the cabbage and set the pan of purple water aside, allowing it to cool. Pour the cooled, cabbage water into a spray mister. Mist the secret message page with the cabbage water. The page and the secret writing will turn different colours, revealing the message.
Tips and warnings
- Write a fake message over the secret message using regular ink or pencil. If the message is intercepted, the perpetrator won't wonder why blank page are being sent out.
- Lemon juice is not entirely imperceptible when written on bright, white paper. Use slightly off-white paper instead.
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