This is a graphic letter that only one person will understand. Secret code, you know. :)
Any kind of organic liquid would work as invisible ink: you migh use milk, vinegar or a juice. I used lemon juice and it was almost invisible when it was dry! The point is that the compounds of ink must burn quickr than paper and leave brown marks on paper. The drawing/text will appear after you hold the piece of paper over a candle for a while.
Have a beautiful day,