This case study documents an example of inquiry learning and teaching during a summer institute for elementary and middle school teachers. A small group constructed an explanatory model for an intriguing optical phenomenon that they were observing. Research questions included: What physics thinking did the learners express? What aspects of scientific inquiry were evident in what the learners said and did? What questions did the learners ask one another as they worked? How did these learners collaborate in constructing understanding? How did the instructor foster their learning? Data sources included video- and audio- tapes of instruction, copies of the participants' writings and drawings, field notes, interviews, and staff reflections. An interpretative narrative of what three group members said and did presents a detailed account of their learning process. Analyses of their utterances provide evidence of physics thinking, scientific inquiry, questioning, collaborative sense making, and insight into ways to foster inquiry learning.