http://www.bbc.co.uk/blogs/thereporters/markeaston/

Perhaps the most famous example of a “nudge” is at Schiphol Airport in Holland. Managers were fed up with the state of the gents. Men were none too accurate in their aim. Warnings and signs did no good so designers tried a “nudge”.

A black fly was etched into the porcelain of each urinal – something to aim at. Spillages were reduced 80%.