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A popcorn kernel is a tiny hard seed. Then it goes BANG and turns into a big white cloud. Where did the cloud come from?
Put popcorn kernels in a hot pan. Put the lid on.
For a while, nothing. Just heat.
Then: pop. Then pop pop pop pop pop. The lid rattles.
When you take the lid off, the little hard seeds are gone. Instead the pan is full of big white fluffy bits.
Nothing was added. It's the same popcorn. It just went bang.
Lots of people think the seed slowly swells, like bread in an oven.
Here is how you can tell that isn't it.
If a kernel has been left out too long, it goes dry inside. Put that dry one in a hot pan and it will never pop. Not in a minute. Not in an hour. It just sits there and burns.
So heat on its own is not enough. Something else has to be inside the seed.
Every good popcorn kernel has a tiny bit of water locked up inside it. You can't see it. It's just there.
You know what heat does to water. A kettle gets hot and steam comes puffing out of the top.
The same thing happens inside the seed. The water gets hot and turns into steam.
But this steam has nowhere to go. The seed has a hard shell all the way round it, and the shell does not let anything out.
Water turns into steam. Steam needs much more room than water does. And inside a popcorn seed, there is no more room.
The steam pushes. And pushes. And pushes.
The shell holds on for as long as it can — and then it can't. It bursts open all at once.
That burst is the pop you hear.
Inside the seed there was a soft hot lump, a bit like very thick porridge. The moment the shell breaks, that lump flies outwards and turns inside out.
Then it cools down and goes hard again. That's your piece of popcorn. It is the inside of the seed, turned out and frozen that way.
Nothing was ever added to the seed. The only thing that changed is that the water inside turned into steam — and steam needs room the seed did not have.
You leave some kernels out on the side for months, so they dry out inside. Then you put them in a hot pan. What happens?
Some seeds pop and some never do. Put each one in the right box, and read what it tells you.
Tap an item, then tap where it belongs
That loud POP you hear — what is actually making the noise?
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That’s the last one in this channel for now. More are on the way.
Back to all episodesA popcorn kernel pops because moisture sealed inside it turns to steam as the kernel is heated; the tough outer hull (pericarp) traps that steam until the pressure ruptures it (USDA / Iowa State University Extension, popcorn science explainers). The starchy inside gelatinises in the heat and expands rapidly when the hull breaks, turning the kernel inside out; it then sets as it cools, which is the white flake you eat (Virot & Ponomarenko, 'Popcorn: critical temperature, jump and sound', Journal of the Royal Society Interface, 2015). Popping depends on the kernel's moisture content: kernels that have dried out below roughly 12-13% moisture do not pop, which is why stale kernels stay hard ('old maids'). Storing popcorn in a sealed container keeps the moisture in (Iowa State University Extension; Purdue University Department of Agronomy). Only popcorn (Zea mays everta) pops well. Its hull is thicker and far less permeable than that of sweetcorn or field corn, so it holds pressure instead of letting steam leak away (Purdue University Department of Agronomy). The pop sound is produced at hull rupture, as the pressurised water vapour is suddenly released — not by the kernel striking the pan or lid (Virot & Ponomarenko, 2015).