You can write all kinds of secret messages that won’t show on paper. Only those persons who know how to make the writing appear will be able to read them. Write your message in milk, lemon juice, or onion juice. You can write it with a thin brush or a clean pen on any kind of paper.
When the milk or juice dries it will not show, and the paper will look blank. The message will show if you hold it over a warm radiator or light bulb.