I pulled this from Drug Monkey, an excellent addiction med research blog:

There was a farmer who grew excellent quality corn. Every year he won the award for the best grown corn. One year a newspaper reporter interviewed him and learned something interesting about how he grew it. The reporter discovered that the farmer shared his seed corn with his neighbors. “How can you afford to share your best seed corn with your neighbors when they are entering corn in competition with yours each year?” the reporter asked.

“Why sir,” said the farmer, “Didn’t you know? The wind picks up pollen from the ripening corn and swirls it from field to field. If my neighbors grow inferior corn, cross-pollination will steadily degrade the quality of my corn. If I am to grow good corn, I must help my neighbors grow good corn.”