The Mpemba Effect is a phenomenon in which hot water freezes faster than cooler water. It seems counter-intuitive, but it’s something that has been observed for millennia.  The story of how it came to be called the Mpemba effect is a great lesson in and of itself. skullsinthestars blog has a great article about it:

“My name is Erasto B Mpemba, and I am going to tell you about my discovery, which was due to misusing a refrigerator.”

With those words, Tanzanian student Erasto Mpemba entered scientific history, and also sparked a scientific mystery and controversy that remains ongoing today, some 40 years later!

The phenomenon Mpemba found is now known as the Mpemba effect, and is the very counterintuitive idea that, under certain circumstances, a quantity of very hot/boiling liquid can freeze faster than an equal quantity of cold liquid!

How is this possible?  The remarkable thing is that nobody really knows, even though the first observations were reported to the scientific community in 1969.  The story of the discovery, and the consequent mystery, is worth a bit of exploration — and the Mpemba effect carries numerous important lessons about the nature and method of scientific discovery.

Mpemba made his accidental discovery in Tanzania in 1963, when he was only 13 years old and in secondary school.  In spite of widespread disdain from his classmates, he surreptitiously continued experiments on the phenomenon until he had the good fortune in high school to interact with Professor Denis Osborne of the University College Dar es Salaam.  Osborne was intrigued, carried out his own experiments, and in 1969 the two published a paper in the journal Physics Education.

For further reading, check out this PhysOrg article about testing the effect.

I suggest a second definition of the Mpemba Effect: the strengthening of resolve in the face of negative criticism.