The counter intuitive question has baffled boffins from ancient Greece up to the modern day. Aristotle wrote in Meteorologica of people first putting water in the sun to later aid the cooling process. More recently, Zimbabwean student Erasto Mpemba noted the paradoxical physics whilst freezing warm ice cream mixture in the lab. First ridiculed for pursuing the findings, he now has the effect named after him.
The answer it seems, is elementary and lies in the forces that keep matter together: bonds. Water, composed of two hydrogen atoms (H) and a single oxygen atom (O), is held together by covalent bonds. However, other forces are at work, weak links between water molecules called hydrogen bonds.
In cold water the molecules are slower moving and more densely packed together, when a hydrogen bond from an H binds to another molecule (O), its covalent bonds stretch out and store energy. These stretched bonds aren’t as common in warm water as the molecules are more spread out. When it comes to cooling time the lack of stored energy in the scattered warm molecules increases the magnitude of energy loss. In some circumstances, the speedy cooling is so dramatic the warm molecules become solid ice before their cooler counterparts.