An ink which leaves no visible mark when used on paper (or other, perhaps treated, surface). The intended recipient must perform some action in order to make the message visible.

Technically, this is a form of steganography; however, it also has a physical aspect (is it really that hard to detect the ink even if invisible?).

You can write with a toothpick dipped in lemon juice; this leaves almost no marks once dry, but the letters can be read by heating the paper over an exposed flame (the marks appear brown; apparently something just gets burnt). If using behind enemy lines, be sure to have a ready explanation for the large amount of lemons and toothpicks carried with you.

Once upon a time, when I was a wee little geek girl, I was fascinated by manuals of cheesy scientific experiments for kids. One such experiment (theoretically to demonstrate capillary action) involved sticking a white carnation into a glass of ink, and watching the bugger change color.

If you put the flower in green ink, it would turn green...
in red ink, it would turn red...
and so forth, very consistently.

So, I eventually formulated the hypothesis that if you stuck a carnation in a glass of invisible ink, it would turn invisible!

Efforts to test this hypothesis were thwarted by the practicality of adults...and my world was the sadder for it...

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