Has anyone ever sworn to you that putting hot water in the freezer makes it turn to ice faster than cold water would? It sounds like the kind of thing your uncle forwards in a group chat, right next to “don’t swim right after eating.” Except this one isn’t quite a myth: the phenomenon is called the Mpemba effect, it has been baffling scientists for over 2,000 years, and a 2025 study has finally started to explain what’s really going on.
The teenager who accidentally discovered the Mpemba effect
The official story starts in Tanzania in 1963. A high school student named Erasto Mpemba was making homemade ice cream with his classmates: the recipe called for boiling a milk-and-sugar mixture, letting it cool down, then freezing it. Racing against his classmates for freezer space, Mpemba skipped the cooling step and shoved his mixture in while it was still hot. To everyone’s surprise, his ice cream set before that of his classmates, who had dutifully waited for theirs to cool first.
When he brought this up with his physics teacher, he got the classic “that’s impossible, it breaks the laws of thermodynamics” brush-off. Lucky for science, Mpemba didn’t let it go, and years later a physicist at the local university, Denis Osborne, decided to actually test it in a lab instead of dismissing it outright. Sure enough, under certain conditions, hot water really could reach freezing point before cold water. The finding was formally published in 1969, and the phenomenon has been known ever since as the Mpemba effect.
A puzzle that already annoyed Aristotle
Here’s the fun part: Mpemba was far from the first person to notice this. Aristotle wrote about it more than two millennia earlier, and it also shows up in the writings of Francis Bacon and Descartes. For centuries, though, nobody took it seriously, because it flies in the face of common sense: if something starts out hotter, it has more distance to cover before reaching 0°C, so it should take longer to freeze, not less. On paper, that logic is airtight.
And that’s exactly the trap: the Mpemba effect is notoriously hard to reproduce consistently in a lab. Sometimes it shows up, sometimes it doesn’t, and it turns out to hinge on details that sound almost trivial — the shape of the container, dissolved gases in the water, convection currents, even where exactly the thermometer sits. That inconsistency led plenty of scientists to suspect for years that the whole thing was an experimental ghost, a measurement artifact rather than real physics.
The 2025 clue: it’s not magic, it’s a molecular obstacle course
This is where it gets genuinely interesting. A 2025 study published in Communications Physics, part of the Nature Communications family of journals, used molecular dynamics simulations to watch the exact moment water begins forming ice — something almost impossible to capture with conventional lab instruments, since it happens at a tiny scale and in fractions of a second.
What the researchers found is that the deciding factor isn’t really the starting temperature, but how long the water gets stuck in a metastable state: a kind of molecular waiting room before it commits to crystallizing into ice. As hot water cools down, it can reorganize its molecular structure in a way that lets it skip part of that wait, while water that started out cold can end up stuck in that in-between limbo for longer before it finally freezes. Critical fluctuations — small, random molecular-level wobbles right before the transition — play a role too, which explains why the effect is so slippery to pin down: it depends on a delicate balance of conditions that shifts from one experiment to the next.
Put another way: hot water isn’t cutting the freezer line by magic — sometimes it just finds a structural shortcut that cold water simply doesn’t have available. And because that shortcut depends on so many variables, it shows up in some runs and not in others, which is, ironically, exactly what had been confusing scientists for decades in the first place.
Should you actually try the Mpemba effect in your freezer?
Go ahead and test it, but don’t expect a guaranteed win: as we’ve seen, the effect is real but extremely sensitive to the exact conditions — container size, water volume, freezer temperature, impurities, and more. So if your tray of hot water ever beats the cold one to the finish line, it wasn’t a fluke or something you did wrong: it’s just physics doing its usual thing, still not fully understood by anyone, ever since the days of Aristotle. If everyday physics fascinates you, you’ll also enjoy why ice is slippery, an answer physics just rewrote after 200 years.