Invisible Ink Pen
When you want to send a spy report without the enemy reading it you write in code. But if someone discovered the code, it will be pretty suspicious. If you want to write something that the enemy will not read, invisible ink is the way to go.
Invisible ink has been used for almost a century. It was first used by the Romans. It was made from milk of the thithymallus plant. In the Revolutionary war, they used sulfate and water to create invisible ink. Some wrote in lemon juice or vinegar. Today we use invisible ink that can be read under a blacklight.
Invisible ink is often written between the lines of letters, notes, and even music sheets. But people are always thinking of new ways to write with invisible ink. In world war I, a woman was discovered carrying an invisible ink message on her back! Once, invisible in was found written on the petals of a flower.
You can make your own invisible ink too.