In short, hot water does freeze sooner than cold water under a wide range of circumstances. It is not impossible, and has been seen to occur in a number of experiments. However, despite claims often made by one source or another, there is no well-agreed explanation for how this phenomenon occurs. Different mechanisms have been proposed, but the experimental evidence is inconclusive. For those wishing to read more on the subject, Jearl Walker’s article in Scientific American is very readable and has suggestions on how to do home experiments on the Mpemba effect, while the articles by Auerbach and Wojciechowski are two of the more modern papers on the effect.