When you get right down to it, there are a lot of substances that can be used as invisible ink. Some invisible inks work by causing slight damage to the paper, so that heating the paper burns the weakened areas to make the writing appear. For other invisible ink, you can write with a chemical that is invisible by itself, but which becomes visible when a second chemical is applied. Here are how-to's for some easy-to-make invisible inks:
You can even make ink that is invisible until it is held under a black (ultraviolet) light. If you have a black light, try writing a secret message using kerosene as an ink.