There are two glasses of water, absolutely identical in all except one thing: the water temperature in the left glass is higher than it is in the right one. Place both cups in the freezer. In what glass water will freeze faster? You can decide that in the glass in which the water was originally colder, but the hot water freezes faster than room-temperature water.
This strange effect is named after a student from Tanzania, who observed it in 1986, when he tried to freeze milk to make an ice cream.
Mpemba effect is possible due to the several factors. Water in a glass of hot water may evaporate and the resulting less water should be freezed. Also, the hot water contains less gas and, hence, it will be easier to freeze it.
Another theory is based on the fact that the weakening of chemical bonds holding the water molecules together.