When students enter the classroom, they often hold prior knowledge or conceptions about the natural world. These conceptions will influence how they come to understand what they are taught in school. Some of their existing knowledge provides good foundation for formal schooling, but other prior conceptions, however, are incompatible with currently accepted scientific knowledge. These conceptions are commonly referred to as "misconceptions." One of the most challenging topics for student conceptual change is bringing them to understand the concept of why things sink and float (WTSF). In this article, the authors present a 10 multiple-choice questionnaire to help teachers diagnose common misconceptions related to WTSF. (Contains 2 figures.)