Iwant you to imagine an experiment. We start with two glasses of water, which are identical to each other in all respects, except one. The only difference between the two is that the water in one glass is hot while the water in the other glass is cold. Now we cool both glasses, using the exact same cooling process, until the water freezes. The question is, which one will freeze first?
Well, obviously it ’s the cold water, right? Let’s say it takes 10 minutes for the cold water to freeze. Then the hot water is going to take some time to cool down to the temperature of the cold water, and then it’s going to take another 10 minutes. So the hot water has to do everything the cold water does, plus a little more, which means that it has to take longer.
It seems to be a simple solution, and you’d be right – most of the time. But not always.
THE STORY OF ERASTO MPEMBA
In 1963, a young Tanzanian high school student named Erasto Mpemba was making ice cream at school in the cookery class. The instructions were to mix boiling milk with sugar, wait for it to cool, and then place the mixture into a freezer. As more and more of his fellow pupils finished the task, Erasto saw the valuable freezer space being used up quickly. So he took a short-cut – instead of waiting for his mixture to cool down, he put his milk in while it was still hot. Sometime later, he opened the freezer to discover something very surprising. For some strange reason, his mixture had frozen into ice cream before those of the other pupils. This seemed impossible, so he asked his physics teacher for an explanation. The teacher dismissed the question, saying that Erasto must have been confused.
Later that year, Mpemba was chatting with a friend of his who made and sold ice cream in the nearby town. His friend