In ancient Greece, in order to preserve the confidentiality of a message, a slave was chosen to deliver same. In order to accomplish this task, the slave's head was shaven cleanly. The message would be tattooed on the crown of the slave's head after which enough time was allowed for the slave's hair to grow back to a reasonable length, whereupon the slave was dispatched to the proposed recipient of the message. When the slave reached the end of his journey, his head was once again shaven, and the message was received.
A tip o' the hat to Sam for his fine story.
