In this article, we address a call by Thompson and Carlson to directly contribute to defining the variation of students' reasoning about varying quantities. We show that students as young as in sixth grade can engage in complex forms of reasoning about multiple quantities in contexts that involve exploring science phenomena using interactive simulations. Specifically, in our data we identified forms of nested, independent, and a form of partial dependent multivariational reasoning. In addition, we noted new multivariational forms that were not identified before in the literature, such as nested and transitive, integrated, and connective multivariational reasoning. We also discuss the role of the science context in students' conceptions of synchronous versus asynchronous variation.