A distinguishing feature of all the formative assessment probes in the "Uncovering Student Ideas" series is that each probe has two parts: (1) a selected answer choice that usually mirrors the research on commonly held ideas students have about concepts or phenomena; and (2) an explanation that supports their answer choice. It is this last part of the probe, the explanation, which provides insight into the extent to which students understand and can use a scientific concept or idea. When students construct explanations to a formative assessment probe, either through writing or science talk, they make their thinking visible to themselves, their peers, and the teacher. However, unless students are explicitly taught how to construct an explanation, their responses to this most important part of a formative assessment probe often fail to provide a sufficient window into their thinking. C-E-O-SE (Commit--Explain--Observe [or Obtain] Information--Scientific Explanation) is a formative assessment classroom technique (FACT) first used during the initial elicitation of students' preexisting ideas (preassessment) and then followed as a post-assessment to check on the extent to which students have developed conceptual understanding of a core idea (Keeley 2014). The scientific explanation (SE) part of this formative assessment strategy is based on the Claims-Evidence-Reasoning (C-E-R) framework described in "What's Your Evidence? Engaging K-5 Students in Constructing Explanations in Science" (Zembal-Saul, McNeill, and Hershberger 2012). In this article, the author demonstrates how the CE-O-SE technique is used to support students' pre- and postinstruction explanations for the formative assessment probes.