Why not become spies for a day and write some secret messages to your friends.
First, you have to make some invisible ink for your secret letter.
Don’t forget that such letters must be written on the back of a real letter or between the lines of another text :).
Use lemon, onion or potato juice for ink. Write your message with a thin brush or a wooden skewer. Wait until the paper is dry.
When you want to read the letter, warm the oven and put the paper in. The letters will soon become visible.