Reasoning about mechanisms is one of the hallmarks of disciplined inquiry in science and engineering, but comparatively little is known about its precursors and development. Children at grades 2 and 5 predicted and explained the motion of simple mechanical systems composed entirely of visible linkages (levers). Students' explanations of device behavior suggested four forms of knowledge: simple recognition of device components, noting of structural relations among components, construction of cause-effect rules derived by observation of regularities in device behavior, and identification of essential system components and interactions among components that accounted for cause-effect rules. Only a few children coordinated multiple essential components to constitute a mechanistic causal scheme. Mechanistic causal schemes, in turn, were associated with successful prediction of the output motion of a system. Device tracing via gesture and talk appeared to support this form of knowledge development, and hence may inform future instructional design. (Contains 17 figures and 3 tables.)