The purpose of explanation is to improve understanding. Left unstated is the question of whose understanding should be improved. On the one hand, scientific output can be thought of as advancing humanity's understanding of some set of phenomena, even if the advance's precise expression are unintelligible to nearly everyone! On the other hand, telling a three-year old that the stork brought his baby sister may yield a completely intelligible and satisficing, albeit ultimately inadequate, feeling of understanding in the child. For any given approach to explanation, the target audience can be thought of as a parameter, with an explanation being tailored to stretch the listener's understanding to the breaking point, but not beyond.