Invisible ink isn't just something you see in James Bond movies or X-Files re-runs. Even Harriet the Spy has her own invisible ink pens. In fact, prisoners of war used their own sweat and saliva as invisible ink. The way invisible ink works depends on the method used to make it. There are several ways to make invisible ink and below you will find a few different recipes for making your own invisible ink and how it works. Next, write using a Q-tip, toothpick or brush on a piece of paper.