Making a treasure map? An in home scavenger hunt?The easiest invisible ink is lemon juice. All you need is lemon juice, a paint brush (or cotton swab) and white paper. and a heat source to make it legible.
Write a message by dipping the brush in lemon juice and drawing on the white paper. Will you say a secret message, leave a code or make a map? Let it dry. Now your ink is invisible.
To make is visible: preheat the oven to 400 degrees. When it is hot enough put your paper on a cookie sheet and place in the oven. After about half an hour your message will become visible.
Paper is made from a compound called cellulose, a bunch of molecules bonded together. The citric acid in lemon juice slowly weakens the bonds between these molecules and frees some of them. When the paper is heated these free glucose molecules react together in a chemical process called Caramelization. This produces new compounds that have a brown color. So it make the ink visible!