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The reason this works is because the highlighter ingredients are fluorescent, so it would have to be a highlighter, and a blacklight. If you want to do this with any old marker, do this, but wash out ALL the ink. You can throw this away. Then saturate the ink cartridge with tonic water. The quinine in the water fluoresces, and should allow you to produce actually invisible messages. This instructable one would still be a little green. I havent tried using it to write messages, so I can't 100% confirm that the concentration of the quinine is high enough to be visible, but I assume it would be. I suppose you could let the mixture evaporate out some water to concentrate it more, but don't boil it, you'll just denature the quinine.
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