A key pursuit of science is to form coherent, mechanistic explanations of natural phenomena. Drawing on a case of fourth graders’ inquiry into sound generation and transmission, we identify instances where the students’ mechanistic reasoning did or did not contribute to their explanation construction process. Our analysis characterizes various ways mechanistic reasoning can contribute, including conjecturing mechanistic details based on various resources, motivating extended coherency checking, and piecing together a fuller explanation. We also single out prematurely established canonical explanations and the scarcity of reasoning resources as two constraining situations. The former allows the unchallenged mechanistic ideas to come back repeatedly. The latter created difficulties in developing anchored conjectures and meaningful coherence checking. Research and pedagogical implications were proposed in the discussion.