The Mpemba effect is the name given to the observation that a liquid (typically water) which is initially hot can freeze faster than the same liquid which begins cold, under otherwise similar conditions. There is disagreement about its theoretical basis and the parameters required to produce the effect.
The Mpemba effect is named after Tanzanian scientist Erasto Bartholomeo Mpemba, who described it in 1963 as a secondary school student. The initial discovery and observations of the effect originate in ancient times; Aristotle said that it was common knowledge.