There is currently debate about the emergence of children's ability to reason about artifacts by reference to their intended design. We present two studies demonstrating that, while 3-year-olds have emerging insights, 4-year-old children display an explicit, well-rounded, adult-like understanding of the way design constrains an artifact's physical…

We are pleased with the commentators' thoughtful reactions to our study, as they make it clear that the study met its major goal--identifying age differences that suggest developmental changes in the learning process and the strengthening of executive processes in the second decade of life.

When a behavior disappears and then resurfaces, developmental psychologists typically look more closely at the behavior to figure out what is different before and after--that is, they increase the grain with an eye toward discovering how the system that generates that behavior has changed. But what ought to count as a U-shaped phenomenon? How…