British intelligence were making some rather creative attempts with invisible ink during the Great War. In 1915, one secret agent was elated to discover a rather convenient way to create one. His classified ingredient? Semen.
The founder and head of MI6 at the time, Captain Sir Mansfield Cumming, reportedly declared “every man his own stylo” when presented with this unique method of creating invisible ink. The only issue? If it wasn’t fresh, the letters would stink really bad!!