> The Mpemba Effect: When Hot Water Freezes First
Sometimes, under the right conditions, hot water beats cold water into ice. Nobody fully agrees on why. I genuinely love that this is still an open question.
The kid who wouldn't let it go
In 1963, thirteen-year-old Erasto Mpemba was making ice cream at Magamba Secondary School in Tanzania. Short on freezer space, he shoved his mix in while it was still hot, instead of waiting for it to cool first like his classmates did. His froze first. When he asked a teacher why, he reportedly got told he was simply wrong about what he'd just watched happen. That was a bad answer, because he kept testing it and kept getting the same result.
He wasn't the first to notice
He was, however, the reason anyone followed up. The observation itself is centuries old. Aristotle wrote in his Meteorology, in the 4th century BCE, that water which has already been heated freezes faster. Francis Bacon noted in the Novum Organum that "water a little warmed is more easily frozen than that which is quite cold." Descartes wrote much the same thing. All three observations sat around unexplained and largely ignored for hundreds of years, because nobody built a real investigation around them.
Making it official
Mpemba eventually got taken seriously by physicist Denis Osborne, who worked with him to actually test the effect properly. They co-published a 1969 paper in the journal Physics Education, titled, with admirable directness, "Cool?" The phenomenon has carried Mpemba's name ever since.
Where it stands now
Still genuinely contested, which is the fun part. Proposed explanations range from faster evaporation in the hot sample (less water left to freeze), to differences in convection currents, to dissolved gas content, to supercooling behavior. A 2016 paper in Scientific Reports directly challenged whether the effect even reproduces reliably under tightly controlled lab conditions at all. So the honest current answer isn't "here's why it happens," it's "it's been observed for over two thousand years, one teenager forced science to actually look at it, and the mechanism is still being argued about." That's a good ratio of "old" to "unresolved."
What happened to the kid
Mpemba didn't stay in a lab. He went on to become Principal Game Officer for Tanzania's Ministry of Natural Resources and Tourism and vice-chairman of the African Forestry and Wildlife Commission, spending most of his career in wildlife conservation rather than physics. He died on May 14, 2023, at age 73, with a genuine, still-unresolved physics phenomenon carrying his name because he refused to accept "you're wrong" as an answer to something he'd watched happen with his own eyes.
