An ice cream shortcut in class
In 1963, Erasto Mpemba was a thirteen-year-old student at Magamba Secondary School in what was then Tanganyika, taking a cookery class that involved making ice cream from a boiled milk and sugar mixture. Pressed for freezer space and time, Mpemba skipped the usual step of letting his mixture cool before putting it in the freezer, placing it in still hot while his classmates waited and cooled theirs first as instructed. When he checked later, his hot mixture had frozen solid before several of his classmates’ properly cooled samples, a result that struck him as strange enough to ask his physics teacher about directly, only to be told the claim was simply impossible and, by implication, that he must have made some kind of error in observation or method.
A teacher who called it impossible
The question did not disappear, and some years later, when the British physicist Denis Osborne visited Mpemba’s school, Mpemba raised it again in front of teachers and classmates who reportedly found the claim absurd on its face, much as his physics teacher had years earlier. Osborne, rather than dismissing the question outright, took it seriously enough to test it, and he invited Mpemba to help carry out formal experiments at the University College in Dar es Salaam to see whether hot liquids really could, under the right conditions, freeze faster than cold ones. Osborne’s own initial scepticism gave way once his experiments appeared to confirm what Mpemba had observed years before in a school cookery class, and the two published their findings jointly in 1969, giving the phenomenon the name it has carried ever since.
A visiting physicist takes it seriously
What holds up in this account is the basic historical sequence: a student’s specific, repeatable classroom observation, initially dismissed by an authority figure, was later tested experimentally by a trained physicist and reported in a peer-reviewed publication under both their names, a genuinely notable outcome given how often dismissed observations from students simply disappear without formal follow-up. It also holds up that the underlying observation was not entirely without precedent; similar claims about heated water freezing more quickly than cold water under some conditions had been recorded far earlier by Aristotle, and later by Francis Bacon and Rene Descartes, indicating this was a persistent if intermittently noted phenomenon across centuries rather than something invented from nothing in a Tanzanian classroom.
A joint paper in 1969
What does not hold up as a settled matter is the phenomenon itself. Since the 1969 paper, researchers have proposed a range of competing mechanisms that might explain faster freezing under hot-start conditions, including differences in supercooling behaviour between hot and cold samples, variations in hydrogen bonding as water cools, differing levels of dissolved gas or minerals, changes in convection currents within the liquid, and the insulating effect of a frost layer forming on colder samples, but no single explanation has become the settled, broadly accepted account. More fundamentally, some researchers have questioned whether the effect is reliably reproducible at all under controlled conditions, and a 2016 analysis concluded there was no evidence supporting a meaningful Mpemba effect specifically in water, a direct challenge to the phenomenon’s basic reality rather than merely to any particular proposed mechanism for it.
Older than the paper that named it
The wider interest in the Mpemba effect lies less in any settled physics and more in what it represents as a case of a genuinely open scientific question that resists easy resolution despite sustained attention, and in the specific circumstance of its naming: a phenomenon carrying the name of the student who first reported it rather than the credentialed physicist who confirmed it, an unusual arrangement in the history of scientific discovery. It also serves as a useful example of how experimental design can dramatically affect reported results in seemingly simple physical measurements, since the sensitivity of freezing time to small variations in container shape, liquid volume, dissolved content, and measurement method appears to be significant enough that different careful experiments can produce genuinely different conclusions about whether the effect exists at all.
Still not settled
This is well worth the time precisely because it does not end in a tidy resolution, and that lack of resolution is itself the interesting part: readers get both a genuinely engaging discovery story, a dismissed student vindicated by a visiting scientist willing to actually run the experiment, and an honest picture of how contested a seemingly simple physical claim can remain more than half a century later. Mpemba himself went on to a career in wildlife management rather than physics, becoming a senior game officer in Tanzania, and died in 2023 still associated primarily with the observation he made as a teenager. Anyone who enjoys a scientific mystery that stays genuinely open, rather than one tied up neatly by the end, will find this a satisfying read.