Why Does Hot Water Freeze Faster Than Cold Water?

Psst! Want £1,000? All you have to do is explain this curious property of water: why does hot water freeze faster than cold water.
The Royal Society of Chemistry is backing the prize:

"Ice cream makers and bartenders alike use the fact that hot water freezes more quickly than cold water every day in their work, but no one really knows why it works. The problem has been around for millennia, with philosophers such as Aristotle and Descartes pondering over it.
"But this effect was reintroduced into the scientific world in 1968 by Erasto Mpemba, a young inquisitive student in Tanzania during a lab session.
"Erasto questioned a teacher on why ice cream froze more quickly when it was boiled, and was quickly told that he was wrong and had probably imagined it. It was only when the teacher performed the experiment himself that he noticed this unusual phenomenon.
"Since the discovery of the effect, scientists have been trying to find out why the phenomenon occurs but remain divided as to what the answer is. It seems that there are lots of possible answers but a conclusive explanation hasn't been produced yet.

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