A student designed a procedure to determine the heat of fusion of ice. She constructed a calorimeter using a polystyrene cup and a thermometer. She weighed the cup, filled it with 150 mL of warm water, then weighed the cup and water together. The temperature of the water was measured, and then ice taken from an ice bath temporarily stored in a –20°C freezer was added to the cup. The cup with water and ice was weighed. The student then covered the cup with a polystyrene lid with two small openings. In one opening, she inserted a thermometer and in the other, she inserted a stirring rod to gently stir the contents of the cup until all the ice was melted. The lowest temperature reached by the water in the cup was recorded. The purpose of weighing the cup and its contents the third (last) time was to