Thanks, yes, I saw that one, too, but I liked the emphasis on relationships. The shorter version is easier to get but it does not explain why this happens. E.g., you can observe some relationship (e.g., test results and a student’s intelligence) and then you target grades. But then you have an incentive to teach to the test, which breaks down the relationship between test results and intelligence. Other people here gave great examples of relationships that can fail.