Freezing depends on impurities (Vilhjalmur Ingi Vilhjalmsson/Getty)

HOT water sometimes freezes faster than cold water – but why? This peculiar phenomenon has baffled scientists for generations, but now there is evidence that the effect may depend on random impurities in the water.

Fast-freezing of hot water is known as the Mpemba effect, after a Tanzanian schoolboy called Erasto Mpemba (see “How the Mpemba effect got its name”). Physicists have come up with several possible explanations, including faster evaporation reducing the volume of hot water, a layer of frost insulating the cooler water, and differing concentration of solutes. But the answer …