Erasto Mpemba was a high school student in Tanzania who noticed, during a project to make ice cream, that heated milk froze more quickly than the cool milk when placed in the freezer. When he asked his science teacher about his observation, his science teacher told him he was wrong. He persisted. Eventually, he came across a college professor (Dr Denis Osborne) who would repeat his experiment. The professor ended up coming to the same results. Under some circumstances, hot water will freeze more quickly than cooler water (under some circumstances, cool water freezers more quickly than warmer water, too).
So, there are two interesting things about this story. First off, this observation was made forty years ago and we still do not understand the effect. Almost every house in America has an ice tray. We know water freezes. However, we cannot be entirely sure how long it will take for water to freeze. There is still a mystery about something as elementary as ice cubes. Secondly, this initial observation was not made by physicists or chemists at the leading research universities, but by a high school student in a developing country.
Youtube: Freezing Water:
How long does it take for water to freeze? A really long time. Check out the six and a half hour long time-lapse video (Cut down to just over seven minutes). The outdoor temperatures varied between -10 and +2 degrees Fahrenheit.
Physics Q & A by the nice folks at the University of Illinois
More about the Mpemba Effect
A somewhat related phenomenon is the Leidenfrost effect. It has to do with the boiling of water.
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