The following is from one of my last semester's textbooks called _Operating Systems: Design and Implementation_ by Andrew S. Tanenbaum (1987, Prentice-Hall). The subject of the chapter is system security and user identification: "...Cats and other animals mark off their territory by urinating around its perimeter. Apparently cats can identify each other this way. Suppose someone comes up with a tiny device capable of doing an instant urinalysis, thereby providing a foolproof identification. Each terminal could be equipped with one of these devices, along with a discrete [sic] sign reading: 'For login, please deposit sample here.' This might be an absolutely unbreakable system, but it would probably have a fairly serious user acceptance problem." And I wouldn't want to clean up the computer lab at the end of the day...
(From the "Rest" of RHF)