The mass of a sample of the material is measured, as is the initial temperature of the water in the calorimeter. The material is then heated, it's temperature measured, and the sample is placed in the water in the inner chamber of the calorimeter. The sample cools as heat is transferred to the water, and the temperature of the water increases. The transfer of heat continues until the sample and the water are at the same temperature. Then the initial and final temperatures of the water are known, and the amount of heat gained by the water can be calculated.