Hot water can freeze faster than cold — and no one fully knows why

It sounds impossible: take two identical cups, fill one with hot water and one with cold, put both in the freezer — and sometimes the hot one turns to ice first. This counterintuitive phenomenon is called the Mpemba effect, named after a Tanzanian student, Erasto Mpemba, who noticed in 1963 that his hot ice-cream mix froze faster than his classmates' cold ones. His teacher laughed at him, but a visiting physicist took the claim seriously and helped publish it.

Scientists have proposed several explanations, and the honest answer is that they still argue about it. One idea is evaporation: hot water loses some mass as steam, so there's less left to freeze. Another points to dissolved gases, which escape from hot water and may change how it cools. A third involves convection currents that move heat around differently in a warm cup. Each explanation works in some experiments but not others.

What makes the Mpemba effect so stubborn is that freezing depends on many tangled variables — container shape, exact starting temperatures, and even the tiny impurities in the water. Because these are hard to control perfectly, some researchers have questioned whether the effect is even real, and results don't always repeat. It's a rare reminder that even everyday water still hides mysteries.