Toilet graffiti

A sign was put on toilet walls that read: 'A local licensed doctor has agreed to donate a set amount of money to the local chapter of the United Way [a heavily publicised American charity organisation] for each day this wall remains free of any writing, drawing, or other markings.' Photograph: Alan George/Getty Images/Flickr RF

The study lasted 50 days and the treatment was completely effective. Yet many-a-public convenience escaped the professor's study.

Graffiti on the wall of a public toilet in Whitstable, Kent Photograph: Michael Jenner/Alamy

Toilet humour, wooden door, Warwick. Photograph: John Robertson/Alamy

Dennis Potter? Really? Photograph: Mika/Corbis

Graffiti in a gents lavatory, Baslow, Derbyshire. Photograph: John Robertson/Alamy

Come in and see my etchings. Photograph: Samba Photo/Getty Images

A toilet wall that almost escaped the graffiti artist's pen. Photograph: John Sanders/Alamy

Big Brother contestant Darren Ramsey left this message on the toilet wall in Manley Airport, Kingston, Jamaica, before a four day tour to promote aids awareness. Photograph: Peter Jordan/Empics

So why did the graffiti stop? Professor Watson speculated that prior to posting the signs, it was not apparent that anyone cared about toilet walls. Photograph: Stefano Amantini/Corbis/Atlantide Phototravel

He offered the alternative explanation that the presence of observers prompted restroom users to refrain from writing on walls. Theories to ponder, perhaps, next time you are faced with literary gems such as 'KD woz here'. Photograph: Public Domain