We describe an experiment with data on the perception of the individual mass of a collection of rocks in comparison to a 100 g brass mass. Students use the logistic differential equation as a reasonable model and estimate parameters. Sensitivity is a subject that invites notions of change, e.g., how sensitive is a system to a slight change in a variable? How discerning are we to change around us? If we could change the wattage to a light bulb some 5 feet from us from 100 to 102 watts, would we n…