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I read a lot of books on Ninjutsu, and the Shinobi as a kid, and I recall that a popular form of invisible ink that they used required the recipient to urinate

I read a lot of books on Ninjutsu, and the Shinobi as a kid, and I recall that a popular form of invisible ink that they used required the recipient to urinate on the paper to reveal the words. Glad this student didn't utilise that form of invisible ink...

FabHK 7y ago
True story: British agents in WW I used sperm as invisible ink, approved by Captain Sir Mansfield George Smith-Cumming of the Secret Service Bureau, under the motto "Every man his own stylo". Apparently some agents didn't use, erm, fresh ink, but previously produced reserves (despite Captain Cummings's decree that "fresh operation was necessary for each letter") and subsequently the practice was discontinued because of the smell. https://en.wikipedia.org/wiki/Mansfield_Smith-Cumming https://en.wikipedia.org/wiki/Mansfield_Smith-Cumming