Science - invisible ink

 

Paper is made of a compound called cellulose. Each molecule of cellulose is made up of thousands of smaller molecules of glucose - a type of sugar, that are bonded together. The citric acid in lemon juice slowly weakens the bonds between the glucose molecules freeing some of them.

When the paper - with no apparent marks on it, is placed in an oven at 170 degrees, these molecules react together in a process called caramelization. This produces new compounds that have a brown colour and make the ink (lemon juice) visible.