Scientists and students often believe that if they just think harder about a problem, a solution will follow. However, thinking about one's thinking, or using metacognition, can be a more productive expenditure of mental energy. Introducing students to metacognition allows them to discover the value of reflection. This article presents a lesson on top-down and bottom-up processing, two of the main ways in which people handle information. Bottom-up processing refers to beginning with a low level (bottom) of thinking to process basic features of stimuli and then arranging these features to form a concept, a high level (up) of thinking. Top-down processing emphasizes how prior knowledge and expectations, which are high levels (top) of thinking, influence how people interpret the low level (down) characteristics of stimuli. Most importantly, the lesson is designed to help students understand why they should proceed slowly and deliberately when making observations in a science lab or when answering questions on a test. (Contains 4 figures.)