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Darwin's classic trial was between two metal buckets of water placed outdoors on a cold, windy night. The one just taken from the fire, froze before the one previously at room temperature. If you put a tiny thermistor into a vessel of cool water in a freezer, you can detect the onset of anomalous convection as it chills below 4 deg C. Colder water in contact with the surface of the vessel rises instead of sinking, so the rate of heat loss at the edge decreases (Newton's Law of Cooling) and the formation of an ice crust at the top and eventually round the sides reduces the rate of heat exchange between the hotter liquid molecules and the ambient air. However if the starting temperature is somewhere in the region of 30 - 60 deg C and the rate of cooling is sufficient (you really do need a metal container and a blast chiller to get a repeatable result) the normal convection flow has sufficient inertia to continue dragging the topmost layer down the side of the vessel below 4 deg C, so the entire mass of water can reach 0 deg C more quickly.