During the Spanish Inquisition, messages were delivered to prisoners via invisible ink. Invisible ink made from human sperm was traced onto hard boiled eggs. The ink passed through the shell to the albumen, and when the egg was peeled the prisoner could read the words. Prison guards allowed visitors in with eggs, seeing no reason they were of any significance. 1915 – the first world war, it is reported that British Intelligence MI6 experimented with invisible ink made from semen.