Students learning to separate variables in order to solve a differential equation have multiple ways of correctly doing so. The procedures involved in "separation" include "division" or "multiplication" after properly "grouping" terms in an equation, "moving" terms (again, at times grouped) from one location on the page to another, or simply carrying out "separation" as a single act without showing any steps. We describe student use of these procedures in terms of Hammer's resources, showing that each of the previously listed procedures is its own "piece" of a larger problem solving activity. Our data come from group examinations of students separating variables while solving an air resistance problem in an intermediate mechanics class. Through detailed analysis of four groups of students, we motivate that the mathematical procedures are resources and show the issues that students must resolve in order to successfully separate variables. We use this analysis to suggest ways in which new resources (such as "separation") come to be. [This paper is part of the Focused Collection on Upper Division Physics Courses.]