Here’s an experiment you can try out at home: Take two cups, fill one with hot water and one with cold water. Label each cup accordingly. Now put both of these cups in a freezer (or fridge). Leave them there for a few minutes, and come back to check on the cups. Keep checking the freezer every few minutes until one of the cups appears to have frozen water in it. Is the frozen water in the hot water cup, or the cold water cup?

Were you surprised by your results? Here’s why:

A seemingly counterintuitive idea, the Mpemba effect describes a process in which, given two quantities of water that do not differ in any way except in initial temperatures, the hot water would actually freeze faster than the cold water.

The knowledge of this phenomenon has been around for quite some time now; it can be traced back to ancient scientists like Aristotle. This effect was rediscovered in the 1960’s by a Tanzanian student named Erasto Mpemba, whom the effect is now named after.

Hot water freezing faster than cold water is also a controversial topic among scientists, not to mention that it could potentially complicate and challenge our current understanding of thermodynamics. There are so many other confounding variables involved, that some say it is almost impossible to determine a single theory or formula that could account for all of them.

The Mpemba effect is still being looked into, and many scientists are unsure of its validity, but hopefully the concept behind water freezing rates has become more clear to you.