In this paper, we show how participants learn to control a simple dynamic stocks and flows task with repeated inflow and outflow decisions. We present the effect that environmental inflow functions of different slopes (positive and negative) have on our ability to control the simple dynamic system. We investigate this slope effect in two experiments with two kinds of functions (linear and non-linear), and we formalize the decision-making process through a System Dynamics model. A process of human and model data fitting common in the Cognitive Sciences helped explain the reasons for the differences found in a system in which the slope of the inflow function is positive in contrast to one in which the slope inflow function is negative (although the total system net flow is the same).