Soap reduced the surface tension of water because only 10 drops of water and soap mixture were added to the penny before the liquid spilled over, compared to 50 drops of water without soap. Sources of error include variations in the volume added with each drop, the time between drops, and the surface of the penny being cleaned as the water/soap mixture is added in each trial. The experiment could be improved by observing other indications of surface tension, including measuring the contact angle.]