There is an apocryphal story about how a horse racing tipster convinced gamblers that he had a crystal ball. He would begin by writing to many of them with his prediction for the next big race. In reality he would tip a different horse to each group of recipients; the next week he would only selectively write to those who had received the last winning tip. After a couple of iterations, the small group of punters who had received only winning tips would be convinced of his genius.

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