Meaningful science education requires an understanding of essential concepts, but it is just as important for scientifically literate persons to use critical thinking as they apply scientific understanding to their lives. Students should learn to use scientific information appropriately to make wise choices and to effectively solve problems that they encounter in life. They must be able to make well-informed judgments about the reliability and accuracy of scientific information that is presented to them. In this article, the author describes an instrument for evaluating critical-thinking skills using science articles in the popular press. The power of this technique lies in its ability to elucidate the kinds of claims and evidence that students rely on when they read science articles. With repeated use, this technique can provide specific information about how student thinking changes throughout a course, offering an effective tool for formative assessment. (Contains 2 figures.)