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How Do Popcorn Kernels Pop?

Jane Lindholm
Inside each corn kernel is a tiny droplet of water. When that water turns to steam, softening the endosperm. Then the pressure blows the husk open.

How do popcorn kernels pop? How do salmon know where to return to spawn? How do rabbits change colors? Why does television fry your brain? How do zippers zip stuff? Who was the fastest runner in the world? In this episode, we'll tackle all these questions!


First, how does popcorn pop and turn into popcorn to eat?

Take a look at a tiny kernel of unpopped popcorn. On the outside, the yellow part is the hard coating known as the husk. It protects the rest of the seeds and doesn't let any moisture in or out.

Inside the husk is a tiny little droplet of water surrounded by something called the endosperm. The endosperm is what you're actually eating when you eat popcorn. Endosperm literally means inside the seed, or within the seed. The word comes from the Ancient Greek language. The endosperm is a starch, or fuel, and protection for the inner part of the seed. In the case of popcorn, it's very hard. Try biting into a kernel of popcorn and you will see. It feels like you're going to break your teeth!

But here's what happens when you heat up a kernel of popcorn — the tiny droplet of water starts to get hot. It gets hotter and hotter until it turns into steam. That steam pushes through the endosperm and turns it kind of soft and that endosperm builds up pressure and finally it has enough pressure that it can explode through that hard outer coating, that husk.

So it explodes outward, breaking through the husk. You hear that process as the popping sound that gives popcorn its name, because the kernel explodes and hits the side of your pot or the bag that it is in in the microwave.

But when it explodes, that soft gelatinous endosperm hits cooler air and it hardens up in whatever shape it has exploded into. So that's why popcorn has all those funny shapes and feels kind of like foam.

Credit Jane Lindholm / VPR
Popcorn, before and after, it has been popped.

Now, popcorn is a special kind of corn. Not every type of corn would do that if you heated it up. So if you dried out a kernel of corn on the cob that you were supposed to eat for dinner and tried to make popcorn, it wouldn't work.

People have been making popcorn for 1,000 or more years. Archeologists even found some popcorn kernels in a cave in New Mexico that are more than 5,000 years old. This species is indigenous to the Americas, that means something that originates or comes from a specific place. Indigenous people in North and South America figured out how to make popcorn many years ago. They showed colonists from Europe. And it took off from there. Popcorn was especially popular in the United States in the 1800s. And even today, the National Popcorn Board says Americans eat about 14 billion quarts each year!

Credit courtesy from parents / VPR
In this episode, we hear from 6-year-old Will in Watertown, MA, 5-year-old Brook in Juneau, AK, and 8-year-old Dieuna from Burlington, VT.

Listen to the full episode for more answers from NPR's Linda Holmes, of the Pop Culture Happy Hour podcast and Riley Woodford, host of Sounds Wild. And YKK explains how their zippers work.

Read the full transcript.

Melody is the Contributing Editor for But Why: A Podcast For Curious Kids and the co-author of two But Why books with Jane Lindholm.
Jane Lindholm is the host, executive producer and creator of But Why: A Podcast For Curious Kids. In addition to her work on our international kids show, she produces special projects for Vermont Public. Until March 2021, she was host and editor of the award-winning Vermont Public program Vermont Edition.
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