Science education reform has long focused on assessing student inquiry, and there has been progress in developing tools specifically with respect to experimentation and argumentation. We suggest the need for attention to another aspect of inquiry, namely "mechanistic reasoning." Scientific inquiry focuses largely on understanding causal mechanisms that underlie natural phenomena. We have adapted an account of mechanism from philosophy of science studies in professional science [Machamer, P., Darden, D., & Craver, C. F., (2000). Thinking about mechanisms. Philosophy of Science, 67, 1-25] to develop a framework for discourse analysis that aids in identifying and analyzing students' mechanistic reasoning. We analyze a discussion among first-grade students about falling objects (1) to illustrate the generativity of the framework, (2) to demonstrate that mechanistic reasoning is abundantly present even in these young students, and (3) to show that mechanistic reasoning is episodic in their discourse. (Contains 7 footnotes and 1 figure.)