What you need:
A toothpick or a paintbrush
A piece of paper
A table lamp or an iron
What to do:
Pass someone a secret messages using lemon juice!
Put your paintbrush or toothpick into some lemon juice and write your secret message on a sheet of paper, with it.
When the juice dries up, it will vanish.
Now give your secret message to your friend and have them heat up the paper using a clothes iron or hold it close to a warm light bulb and watch your writing magically appear again!
Be very careful while warming the sheet of paper with your message.
Note that this trick will not work if you use LED lights.