Young children are surprisingly judicious imitators, but there are also times at which their reproduction of others' actions appears strikingly illogical. For example, children who observe an adult inefficiently operating a novel object frequently engage in overimitation, persistently reproducing the adult's unnecessary actions. Though children readily overimitate irrelevant actions that even chimps ignore, this curious effect has previously attracted little interest; it has been assumed that children overimitate not for theoretically significant reasons, but rather as a purely social exercise. This dissertation, however, challenges this view, presenting the first evidence that overimitation reflects a more fundamental cognitive process. A series of seven experiments shows that children who observe an adult intentionally manipulating a novel object have a strong tendency to encode all of the adult's actions as causally meaningful, implicitly revising their causal understanding of the object accordingly. This automatic causal encoding process allows children to rapidly calibrate their causal beliefs about even the most opaque physical systems, but it also carries a cost. When some of the adult's purposeful actions are unnecessary---even transparently so---children are highly prone to mis-encoding them as causally significant. The resulting distortions in children's causal beliefs are the true cause of overimitation, a fact that makes the effect remarkably resistant to extinction. Despite countervailing task demands (Experiments 1-5), time constraints (Experiments 2 and 4), direct warnings (Experiment 3), and even extreme competitive pressure (Experiments 4 and 5), children are frequently unable to avoid reproducing the adult's irrelevant actions because they have already incorporated them into their representation of the target object's causal structure. At the same time, however, overimitation is not indiscriminate. Children will not causally encode actions that appear to be unintentional (Experiment 6), or which imply a violation of the contact principle (Experiment 7). These boundary conditions, logically coherent in terms of overimitation's posited learning function, reinforce the conclusion that the effect reflects much more than a simple social game. In fact, the evidence presented here argues that overimitation represents an important window onto the structure of children's emerging causal understanding.