Sometimes, hot water freezes faster than cold water. It goes against everything you might assume, but it's true—minds as great as Aristotle, Descartes, and Francis Bacon have described it happening. The counterintuitive phenomenon didn't come to the attention of modern science, however, until a 13-year-old Tanzanian student named Erasto B. Mpemba noticed it himself while making ice cream. Because he was in a rush to get his ice cream in the freezer before other students had taken up all the space, Mpemba decided to put his boiling milk in without letting it cool first. Surprisingly, he found that his milk froze long before anyone else's. Mpemba asked his teachers why this might be, and most told him he must be mistaken. But a visiting physics professor named Dr. Denis Osborne thought about his question and asked a lab technician to test Mpemba's claim. Sure enough, hot water froze faster than cold in experiment after experiment, and in 1969, Osborne and Mpemba published a paper about the phenomenon.
Even several decades later, scientists still struggle to explain the mechanism behind the Mpemba effect. The effect doesn't happen every time, making it difficult to study. The most popular hypothesis is that hot water evaporates more quickly, so it loses more mass and needs to lose less heat to freeze. But scientists have observed the Mpemba effect in closed containers with no evaporation. Supercooling, the idea that dissolved gases in the water might speed the freezing process, is also a possibility. In 2013, the media claimed that a team of researchers from Singapore had found the answer: the bonds between molecules in boiling water are more flexible and ready to give up energy (in the form of heat) than those in cool water. Unfortunately, scientists don't find their explanation as airtight as the public does. Learn more about the states of water with the videos below.