HOW is it possible for hot water to freeze more quickly than cold? This peculiar phenomenon, first noticed by Aristotle in the 4th century BC, has baffled scientists for generations. But a South African physicist now claims that the answer lies in water's ability to remain liquid below its normal freezing point. "It's all to do with supercooling," says David Auerbach, who works at the Max Planck Institute for Fluid Dynamics in Göttingen, Germany.
The phenomenon is today known as the Mpemba effect, after the Tanzanian schoolboy Erasto Mpemba. In the 1960s, Mpemba became a laughing stock after telling his science teacher he could make ice-cream mixture freeze faster by warming it before putting it into the freezer.
Since then, many explanations have been proposed (see The Last Word, this issue). Mpemba himself ruled out two of the most obvious. One suggestion was that the hot mixture melted a layer ...
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