An investigation into the reproducibility and possible causes of the Mpemba effect has been performed. The Mpemba effect is the name given to the common observation by non-scientists that initially hot water appears to freeze faster than initially cold water. Previous scientific studies of this effect have found conflicting results. These discrepancies appear to be due mainly to inconsistent definitions of freezing based on visual observation. We have investigated the Mpemba effect by continuously monitoring the temperature of a sample of water to determine the amount of time needed for the water to turn copletely to ice, as indicated by the temperature falling below its freezing temperature. We have successfully observed the effect repeatedly and have found it to be dependent on the history of the sample rather than the sample temperature when placed into the freezer. Water briefly heated to 100 �C then cooled to room temperature froze approximately 50% faster than water which had not been heated. The effect on the freezing time of increasing or decreasing the amount of dissolved gas in the water will also be discussed.