This study examined teachers' expressions of the dynamic characteristics of open inquiry, which emphasize the scientific, continuous, and flexible thinking and affective aspects associated with the inquiry process. Metacognitive prompts supported teachers' reflections as they engaged in an open inquiry process. Using both qualitative and quantitative methods, we analyzed teachers' reflections and characterized their expressions of the dynamic characteristics of open inquiry. The results indicate that metacognitive prompts focused teachers' attention to the characteristics of open inquiry, which were expressed in their reflections. These expressions varied according to the metacognitive prompts used to support the teachers. The results have implications for teachers' professional development, specifically for the design of metacognitive prompts to support teachers in internalizing the dynamic nature of open inquiry.