There are many meanings of "scientific reasoning" or "scientific inquiry" in use, and many corresponding orientations toward its enhancement and tracking. Deciding what these terms mean once and for all is an elusive and likely chimerical goal. However, setting down some core models might help in being clear on where different researchers stand and how their work relates. The author's intention in this article is to posit two core models that are neither contradictory nor exhaustive. Kuhn and Pease's "inquiry skills" focus on one type of inquiry, investigating how students progress in understanding "Related Variation." In this article, the author explains this category in stages, starting from a familiar, classical example. (Contains 3 footnotes.)