Residents of the St. Pauli neighborhood of Hamburg had an unusual problem. The area is home to many popular pubs and bars that draw 20 million drinkers each year. This all was fine except for one thing: A certain number of men did not make use of the traditional amenities to relieve themselves. The result was an unpleasant and lingering odor, that being urine.
The neighborhood hit on a solution involving the psychological principle of aversive conditioning -- a variation on the work of Ivan Pavlov, who won a 1904 Nobel Prize for his work demonstrating that dogs could be trained to salivate at the sound of a bell after the sound had been associated with the arrival of tasty dog treats.
The agent of the Hamburg conditioning project was not a bell but a type of nautical paint, typically applied to the hulls of ships to prevent rust.
Pavlov would be proud.