Here’s the weird part: if you put a container of hot water and a container of cold water in the freezer at the same time, the hot one can sometimes freeze first. This is called the Mpemba effect, and it’s been puzzling scientists for decades. But — and this is important — it doesn’t always happen.
According to MIT engineers, one reason this can occur is that your freezer actually works harder when you put something hot inside it. The rate of heat transfer — basically how fast the freezer pulls warmth out — starts off higher for the hot water. So in some situations, that faster early cooling gives the hot water a head start it never fully loses.
Another explanation from Quanta Magazine: hot water evaporates faster than cold water. That means some of the hot water literally turns to vapor and escapes before it freezes, leaving less water behind. Less water means less time to freeze. Cold water could also contain more dissolved gases, which may lower its freezing point and slow things down.
But here’s the honest part — this doesn’t work under all conditions. Scientists at UC Riverside point out that if the hot water starts very close to boiling and the cold water starts almost at freezing already, the cold water will clearly freeze first. The effect is real, but it’s not a guaranteed trick every time.
The single biggest takeaway: science isn’t always intuitive. Something that sounds obviously wrong — hot freezing faster than cold — can actually be true sometimes, depending on factors like evaporation, gas content, and how hard your freezer has to work. If you want to try it yourself, use identical containers, similar amounts of water, and vary the starting temperatures. Just don’t expect it to work perfectly every single time.