This is why World War I-era British spies used semen as invisible ink
The first head of Britain's secret service — which would one day be called MI6 — carried a swordstick, drove a personal tank, and would sometimes stab his wooden leg with a pen just to see how people reacted.
If that wasn't enough to make him eccentric, his department also discovered that semen makes an excellent invisible ink.
It's probably best not to ask why. Or how.
No one actually knows which British agent was the one who came up with the idea, but the book "Six: The Real James Bonds 1909-1939" notes that his fellow spies made so much fun of him that he had to be transferred to another office.
His name was — no joke — Captain Sir Mansfield Cumming and his agents lived by the motto, "Every man his own stylo."
The truth was, British spies were searching for the perfect invisible ink during World War I and thought natural fluids were the ideal. The major issue with using semen to write letters? The smell eventually becomes very distinctive.
Cumming ruled that agents abroad using this method of secret messaging ensure their ink was fresh for every letter.
"Bill, what does this smell like to you?"
The book details an agent in Copenhagen, a Maj. Richard Holme, who apparently kept a ready supply on hand.
"...his letters stank to high heaven and we had to tell him that a fresh operation was necessary for each letter."
In "Prisoners, Lovers, and Spies: The Story of Invisible Ink," Kristie Macrakis writes that Cumming began inquiring about the use of bodily fluids as invisible ink as early as 1915 and told Walter Kirke, Deputy Head of Military Intelligence that he thought the best invisible ink was indeed semen.
"Use a pen."
Semen does not react to the iodine vapor test, a method that then turned all known invisible inks brown. This was particularly attractive to the spy agency, but unfortunately (for spies — not for those concerned with hotel cleanliness) heat develops semen ink and it appears in ultraviolet light.