We describe an approach that uses causal reasoning and geometric reasoning to construct explanations for the purposes of the geometric features on the parts of a mechanical device. To identify the purpose of a feature, the device is simulated with and without the feature. The simulations are then translated into a “causal-process” representation, which allows qualitatively important differences to be identified. These differences reveal behaviors that the feature causes to occur and those it prevents from occurring. The focus of this paper is geometric reasoning techniques that reveal causal relationships between the caused and prevented behaviors. For example, these techniques can determine if a particular caused behavior is responsible for preventing a particular prevented behavior.