144 children aged 5 to 12 years made initial predictions about the speeds of a heavy and a light ball rolling down a slope. They were then asked to consider how changing the incline height would impact the initial predictions. The findings illustrate a shift from rigid differentiation to more flexible knowledge structures. While perceptions changed with increasing age from light-as-faster to heavy-as-faster, younger children were also less likely to believe that any other incline steepness could conceivably lead to a different outcome. Older children, on the other hand, showed a heightened awareness of how changing incline heights could allow for alternative motion patterns. The study adds to current understanding of conceptual development. It expands on the debate between knowledge-in-pieces and knowledge-as-theory, concluding within its constrained scope that development of scientific knowledge about object motion possibly occurs in a transition from pieces to theory. Consequentially, the paper also considers implications for early science education.