Contextual support for performance and understanding plays an important role in learning and teaching. This study investigated the temporal course of the effects of support--how it affects complexity and correctness of judgments about density in kindergarten (n = 35) and second-grade (n = 29) children. In the experimental group, a teacher provided support through modeling more complex reasoning about why objects sink or float. Children's complexity judgments increased sharply with support compared to no support, although their predictions about whether objects would sink or float were mostly correct from the start. Following the support event, children showed a sudden jump in complexity of explanations, which was transient for most children, who showed either rapid decrease or some oscillation and then decrease. A few sustained the high-level explanation after the jump, showing robust knowledge. That is, patterns of performance across trials were primarily nonlinear, following mostly cubic or quadratic change. In addition, second-graders had a more complex understanding of density than did kindergarteners. Findings indicate that children's concepts are dynamic rather than static, as evidenced by the strong but transient effects of support for most students. To move from transient to robust knowledge requires the building of knowledge and skill over time.