As an exchange student, I live in on campus accommodation that is run by the university. In an effort to decrease water usage by students, the university put up posters in all of the bathrooms. The poster is put just above the sink, so as to not be missed by any user of the bathroom.
This “cut the flow” poster gives examples of all the ways students can decrease their water usage. For example, having shorter showers, not running the tap when you brush your teeth, and taking less baths. Most importantly, the poster states that if you save water, your residence block could win a prize. It does not specify what prize, just a prize. This way, the mystery prize can be applicable to everyone on the block. If you did not care for the supposed prize, you would not have an incentive to save water. Therefore, a vague prize is declared. Research done by Deci (1971) indicates that people are very motivated by rewards. Therefore, students may be more motivated to help the environment with the possibility of getting rewarded by the secret prize.
Deci, E. L. (1971). Effects of externally mediated rewards on intrinsic motivation. Journal of Personality and Social Psychology, 18, 105-115.