The Schoolboy Who Discovered a Physics Paradox
In 1963, a 13-year-old Tanzanian student named Erasto Mpemba was making ice cream in his school's cooking class in Magamba, Tanzania. When space in the freezer ran out, he skipped the cooling step and put his hot milk-and-sugar mixture straight in. His froze first.
He told his physics teacher. The teacher said he was confused, his observation was clearly impossible. His classmates laughed. The incident stuck. For years, other students at his school called his blunder out by name: "That is Mpemba's physics."
A few years later, Denis Osborne, a physicist from University College in Dar es Salaam, came to give a guest lecture at Mpemba's new school. At the end, Mpemba asked him directly: "If you take two beakers with equal volumes of water, one at 35°C and one at 100°C, and put them in a freezer, the one that started at 100°C freezes first. Why?" The room erupted. Osborne was caught off guard, but instead of dismissing the question, he promised to test it.
Back in his lab, Osborne asked a young technician to run the experiment. The technician confirmed hot water froze first, then added: "But we'll keep repeating the experiment until we get the right result." Repeated tests gave the same result every time. In 1969, Mpemba and Osborne co-published their findings in the journal Physics Education under the title "Cool?" The phenomenon became known as the Mpemba Effect.
The mechanism is still not settled. Evaporation, dissolved gases, convection currents, hydrogen bond dynamics, each has its proponents, none has closed the debate. A 2024 paper in the Journal of Chemical Physics proposed yet another molecular explanation. Erasto Mpemba, who spent his career as a game warden in Tanzania, died in May 2023. The question he asked in a school kitchen sixty years ago is still officially open.
He was laughed at twice for the same question, once as a child, once as a teenager. Physics has been trying to explain it ever since.