British spies were experimenting with all kinds of invisible inks during the first World War. In 1915, one spy was quite excited to discover a rather handy way to create one. His secret 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 novel method of concocting invisible ink. The only problem? If it wasn't fresh, the letters would become rather pungent.
http://factually.gizmodo.com/british-spies-used-semen-as-invisible-ink-during-wwi-1614656875