science
A schoolboy in Tanzania once startled his teacher with a claim about a hot mixture freezing, and physicists have argued over it ever since.
The Mpemba effect describes situations where warmer water can freeze faster than cooler water under the same conditions, and it is real enough that curious students and scientists keep re-observing it. Several mechanisms likely combine to produce it: hot water evaporates faster, losing mass in a way that speeds cooling; it holds less dissolved gas; and it can trigger stronger convection currents that move heat away more efficiently.
There is no single dominant explanation, which is why the effect remains debated. It is not universal either, it depends heavily on container shape, temperature, and stirring, so heating a glass of water will not reliably win a race against a cold one.
The effect is named for Erasto Mpemba, a Tanzanian student who noticed his hot ice-cream mix froze before his classmates' cooler mix and pushed a skeptical teacher to investigate, turning a schoolboy's observation into a genuine physics puzzle.
science
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