The Mpemba Effect

That's one I missed.

I don't know how, since Aristotle described it around 300-350 B.C.

Of course, the phenomenon wasn't named that back then, which is probably why it didn't ring a bell when I happened on it in Anahad O'Connor's "Really?" column in the March 18, 2008 New York Times Science section.

He wrote, "Under the right circumstances... hot water can freeze more quickly than cool water. Part of the reason appears to be that hotter water loses mass to evaporation, and because it has less mass, less energy is needed to freeze it."

"That phenomenon... described as far back as 350 B.C. by Aristotle... is now known as the Mpemba effect... after Erasto B. Mpemba, a student who noticed it in 1963 while using boiled milk to make ice cream and reintroduced the concept to the scientific literature."

From Wikipedia: "Mpemba first encountered the phenomenon in 1963 in Form 3 of Magamba Secondary School, Tanzania when freezing hot ice cream mix in cookery classes and noticing that they froze before cold mixes. After passing his O-level examinations, he became a student at Mkwawa Secondary (formerly High) School, Iringa, Tanzania. The headmaster invited Dr. Denis G. Osborne from the University College in Dar Es Salaam to give a lecture on Physics. After the lecture, Erasto Mpemba asked him the question, "If you take two similar containers with equal volumes of water, one at 35°C and the other at 100°C, and put them into a refrigerator, the one that started at 100°C freezes first. Why?" only to be the ridiculed by his classmates. After initial consternation, Dr. Osborne confirmed Erasto's finding and they published the results together in 1969."

A much, much more detailed 1998 treatment appears here.

A paper published in 2006 in the American Journal of Physics explained how it happens; the abstract of that report follows.

The Mpemba effect: When can hot water freeze faster than cold?

We review the Mpemba effect, where initially hot water freezes faster than initially cold water. Although the effect might appear impossible, it has been observed in numerous experiments and was discussed by Aristotle, Francis Bacon, Roger Bacon, and Descartes. It has a rich and fascinating history, including the story of the secondary school student, Erasto Mpemba, who reintroduced the effect to the twentieth century scientific community. The phenomenon is simple to describe and illustrates numerous important issues about the scientific method: the role of skepticism in scientific inquiry, the influence of theory on experiment and observation, the need for precision in the statement of a scientific hypothesis, and the nature of falsifiability. Proposed theoretical mechanisms for the Mpemba effect and the results of contemporary experiments on the phenomenon are surveyed. The observation that hot water pipes are more likely to burst than cold water pipes is also discussed.