What does tossing one coin into the Trevi Fountain supposedly guarantee?

Trevi Fountain is the most beautiful fountain in Rome. Measuring some 20 meters in width by 26 meters in height, Trevi Fountain is also the largest fountain in the city. There is a popular myth associated with throwing coins in the waters of the fountain. The myth, originating in 1954 with the movie "Three Coins in the Fountain," goes like this: If you throw one coin: you will return to Rome. If you throw two coins: you will fall in love with an attractive Italian. If you throw three coins: you will marry the person that you met. In order to achieve the desired effect, you should throw the coin with your right hand over your left shoulder. An interesting statistic is that approximately a million euros worth of coins are taken from the fountain each year. Since 2007 this money has been used to support good causes.

Source: Civitatis Rome