Can Hot Water Freeze Faster Than Cold Water?

This story provides a dramatic parable cautioning scientists and teachers against dismissing the observations of 
non-scientists and against making quick judgements about what is impossible.

In 1963, Mpemba was making ice cream at school, which he did by mixing boiling milk with sugar.  He was supposed to 
wait for the milk to cool before placing it the refrigerator, but in a rush to get scarce refrigerator space, put his 
milk in without cooling it.  To his surprise, he found that his hot milk froze into ice cream before that of other students.
He asked his physics teacher for an explanation, but was told that he must have been confused, since his observation was impossible.

Mpemba believed his teacher at the time.  But later that year he met a friend of his who made and sold ice cream in 
Tanga town.  His friend told Mpemba that when making ice cream, he put the hot liquids in the refrigerator to make them 
freeze faster.  Mpemba found that other ice cream sellers in Tanga had the same practice.

Later, when in high school, Mpemba learned Newton's law of cooling, that describes how hot bodies are supposed to cool 
(under certain simplifying assumptions).  Mpemba asked his teacher why hot milk froze before cold milk when he put them 
in the freezer.  The teacher answered that Mpemba must have been confused.  When Mpemba kept arguing, the teacher said, 
"All I can say is that is Mpemba's physics and not the universal physics" and from then on, the teacher and the class would 
criticize Mpemba's mistakes in mathematics and physics by saying "That is Mpemba's mathematics" or 
"That is Mpemba's physics." But when Mpemba later tried the experiment with hot and cold water in the biology laboratory 
of his school, he again found that the hot water froze sooner.

Earlier, Dr Osborne, a professor of physics, had visited Mpemba's high school.  Mpemba had asked him to explain why hot 
water would freeze before cold water.  Dr Osborne said that he could not think of any explanation, but would try the 
experiment later.  When back in his laboratory, he asked a young technician to test Mpemba's claim.  The technician 
later reported that the hot water froze first, and said "But we'll keep on repeating the experiment until we get the 
right result." However, repeated tests gave the same result, and in 1969 Mpemba and Osborne wrote up their results.


[Extracted from http://math.ucr.edu/home/baez/physics/General/hot_water.html]
[Also found at: http://www.bbc.co.uk/gloucestershire/weather/2003/12/ice.shtml]