The Mpemba effect is the observation that hot water can sometimes freeze faster than cold water. A 2013 paper by Xi Zhang, Chang Q Sun and colleagues proposes that the answer lies in the O:H-O bond between water molecules. Heating lengthens the weak O:H link but shortens and stiffens the covalent H-O bond, storing energy in it. When the water is cooled, the H-O bond releases this energy at a rate that rises exponentially with how much was stored, so warmer water can lose heat faster.
via O:H-O Bond Anomalous Relaxation Resolving Mpemba Paradox | arXiv