Formative assessment probes can be effective tools to help teachers build a bridge between students' initial ideas and scientific ones. In this article, the authors describe how using two formative assessment probes can help teachers determine the extent to which students make similar connections between developing a concept of matter and a concept of rocks. Both probes require students to consider the properties of size, weight, shape, texture, or form in developing a concept of "matter" or "rock." (Contains 2 figures.)