The Mpemba Effect is a special phenomenon where hot water freezes faster than cold water. The discovery of this effect was made by a high school student named Mpemba in Tanzania, Africa in 1969. He noticed this phenomenon while making ice cream and was curious enough to make note of it. His teachers did not believe it was possible, and it took several years until university professors finally accepted his discovery.

 (source: http://www.school-for-champions.com/science/mpemba.htm)