Half a lemon
Lamp or other light bulb
What to do:
Squeeze some lemon juice into the bowl and add a few drops of water.
Mix the water and lemon juice with the spoon.
Dip the cotton bud into the mixture and write a message onto the white paper.
Wait for the juice to dry so it becomes completely invisible.
When you are ready to read your secret message or show it to someone else, heat the paper by holding it close to a light bulb.