In this article the authors present an analogy, "Apple Activity," designed to help science teachers and students reflect upon science instruction and learning at the three conceptual levels of scientific understanding--macroscopic level, sub-microscopic/particle level, and symbolic level. Analogies are useful instructional tools that improve individuals' understanding by anchoring abstract concepts to concrete and familiar entities. This analogy provides a simple and familiar situation designed to help science teachers reflect on their students' perspectives. Too often science instructors, including the authors, teach science through traditional lectures that mainly address the symbolic and macroscopic levels. This approach is logical for science teachers who are experts but it can be confusing for students who are relative novices. The "Apple Activity" creates an analogy that helps science instructors to "see" the differences between experts and novices. After the authors introduce the "Apple Activity" they discuss how it connects the Three Conceptual Levels of Scientific Understanding to science content and effective teaching practices. (Contains 11 figures.)