In this regular "American Educator" column, findings from the field of cognitive science that are strong and clear enough to merit classroom application are considered. Individuals vary in their views of what students should be taught, but there is little disagreement on the importance of critical thinking skills. In free societies, the ability to think critically is viewed as a cornerstone of individual civic engagement and economic success. In this column, Daniel Willingham offers a commonsensical view. One is thinking critically if (1) the thinking is novel--that is, not simply drawing a conclusion from a memory of a previous situation; (2) the thinking is self-directed--that is, not merely executing instructions given by someone else; and (3) the thinking is effective--that is, it respects certain conventions that make thinking more likely to yield useful conclusions. These would be conventions like "consider both sides of an issue," "offer evidence for claims made," and "don't let emotion interfere with reason." This column focuses on this third characteristic and shows how what constitutes effective thinking varies from domain to domain.