Making an invisible message is easy. All you'll need is a piece of paper, lemon juice and a something to write with, like a small artist's brush or a straw. Dip your writing utensil into the lemon juice and write a message on the piece of paper, then let the "ink" dry. Once it is totally dry, your message will no longer be visible. The recipient of your message can make it visible again by placing it over a flame or other heat source such as a light bulb.
Lemon juice is a weak acid. When it is placed on paper, it weakens the paper's fibers in that location. Because of this, when you subject the paper to a heat source, the weaker parts of the paper --the message -- turn a darker color first, making it easy for you to read the once-hidden message.
While making invisible ink is a good science experiment for a class or for a science fair project, you may want to have students take the project further by looking at how different concentrations of lemon juice work in creating invisible ink. Also, you can explore whether different weights of paper will make the message more or less visible. You can have students look at how letting the "ink" set for different periods of time affects its visibility when subjected to a heat source.
In addition to lemon juice, you can also use other weak acids as vinegar, apple juice, grape juice and even milk. Try one or more of these liquids and note any differences in the liquids when used as inks. Be sure to note any differences in color.