During World war I, British spies used semen as invisible ink. Sir Mansfield Cumming invented this method.
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British spies experimented with all kinds of invisible inks. Cumming, founder and head of MI6 then, presented this method and declared that “every man his own stylo.”
However, the only issue was that the letters were rather pungent.
According to reports, one spy from Copenhagen used old semen stored in a bottle and his letters smelt nasty.
Since then, Cumming insisted that “fresh operation was necessary for each letter.”
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