I heard this from a friend who is devoted to the Irish goddess Brigid. In the old days, a well was an open pool that formed around a spring. They were holy places, where water came magically from the earth. The custom developed of offering the spirit of the well a coin, with the spirit granting a wish to the gift-giver in return. Today, you can toss a coin in a fountain or a creek or stream. Just make a wish beforehand, and always leave without looking back.