We review the Mpemba effect, where intially hot water freezes faster than initially cold water. While the effect appears impossible at first sight, it has been seen in numerous experiments, was reported on by Aristotle, Francis Bacon, and Descartes, and has been well-known as folklore around the world. It has a rich and fascinating history, which culminates in the dramatic story of the secondary school student, Erasto Mpemba, who reintroduced the effect to the twentieth century scientific community. The phenomenon, while simple to describe, is deceptively complex, and illustrates numerous important issues about the scientific method.
This was one of those water anomalies that was around for centuries but defied scientific common sense. It should be impossible for the same mass at high energy to freeze first. The student Mpemba found he could freeze ice cream faster by heating it first. He was met with skepticism. But he persisted until there was an investigation. After hundreds of years of thinking this was nothing but superstitious folklore, it turned out to be real.