This article explores ways of characterizing different dimensions and levels of scientific reasoning in early elementary school children, in the context of open explorations. The article focuses on children's performance on three probes which involve using evidence to generate and evaluate knowledge claims. A number of dimensions have been used to characterize children's approaches to exploration and knowledge construction, which demonstrate the interrelationship between conceptual knowledge and scientific reasoning. Children differed markedly across these dimensions, yet individual children were relatively consistent in their approach to the tasks. The major differences in performance are linked to fundamental distinctions in the way ideas are viewed in relation to evidence.