WTF Fun Fact – Semen As Invisible Ink

During World War I, members of Britain’s MI6 discovered that semen could be used as an invisible ink. The bodily fluid didn’t react with the traditional detection method of iodine vapor. They stopped trialing it when it was discovered that stale semen could be detected by its smell. – WTF Fun Facts

Source: https://www.telegraph.co.uk/news/uknews/8015180/MI6-used-bodily-fluids-as-invisible-ink.html