It sounds backward: pour one tray of hot water and one tray of cold water into a freezer, and the hot water may turn to ice first. This surprising behavior is often called the Mpemba effect, after a Tanzanian student, Erasto Mpemba, who noticed it while making ice cream in the 1960s. Scientists have debated it for decades because it does not happen every time and depends strongly on the setup.

Several processes can give hot water an advantage. Evaporation is one: warm water can lose more mass before freezing, leaving less water to turn into ice. Convection is another: hotter water circulates more vigorously, which can spread cooling through the container in a different way. Heating water can also drive out dissolved gases and change how ice crystals begin to form. In some freezers, a warm container may slightly melt frost beneath it, improving contact with the cold shelf and helping heat escape faster.

The key phrase is “under certain conditions.” Container shape, water depth, starting temperatures, freezer airflow, impurities, and whether the water is disturbed can all matter. In many ordinary tests, cold water will freeze first, exactly as common sense suggests. But when the conditions line up, the hotter sample can cross the finish line earlier.

The Mpemba effect is a useful reminder that everyday science can be more subtle than simple rules. “Hot cools faster” is not the whole story, and neither is “cold always freezes first.” Freezing involves heat transfer, evaporation, nucleation, and the environment around the water. That complexity is why the effect remains fascinating: it turns a kitchen-table question into a lesson about how nature often rewards careful observation over quick assumptions. Next time you fill an ice tray, remember that temperature is only one part of the freezing puzzle, not the answer.