This qualitative video study explores how two elementary school teachers taught for conceptual understanding throughout different phases of science inquiry. The teachers implemented teaching materials with a focus on learning science key concepts through the development of word knowledge. A framework for word knowledge was applied to examine the students' level of word knowledge manifested in their talk. In this framework, highly developed knowledge of a word is conceptual knowledge. This includes understanding how the word is situated within a network of other words and ideas. The results suggest that students' level of word knowledge develops toward conceptual knowledge when the students are required to apply the key concepts in their talk throughout all phases of inquiry. When the students become familiar with the key concepts through the initial inquiry activities, the students use the concepts as tools for furthering their conceptual understanding when they discuss their ideas and findings. However, conceptual understanding is not promoted when teachers do the talking for the students, rephrasing their responses into the correct answer or neglecting to address the students' everyday perceptions of scientific phenomena.