Have you ever wanted to write a secret message where nobody could look in except you and the person you are writing to because it is invisible to all? If you do we can tell you how...
Lemon juice, and any other things made from plants, are called organic chemicals and contain carbon. When organic chemicals burn the carbon, which is black, breaks off from the molecule, like wood turns black when it is burned. Instead of lemon juice, you could use any fruit juice or tree sap (don't cut a living tree to get the sap), but this experiment works best with colourless or light-coloured liquids.