DAVID GREENE, HOST:

It's Oscar time, which means I'm thinking about going to the movies. A trip to the movies, though, not cheap these days. Twelve-dollar tickets, four to $5 for a soda or a box of candy. And a large-popcorn can run you upwards of 10 bucks. Turns out, while you're munching on that overpriced All-American popped finger food, you're actually snacking on a truly ancient treat. Archeologists have uncovered popcorn kernels that are 4,000 years old. And they're so well preserved, they could still pop.

NPR's Adam Cole explores the history and the science of his favorite movie snack.

ADAM COLE, BYLINE: Dolores Piperno studies the foundation of empires. And by that she means corn.

DOLORES PIPERNO: When you have a very highly productive crop like corn, that makes the rise of high civilizations possible.

COLE: Piperno works at the Smithsonian's Tropical Research Institute in Panama, cultivating corn's wild ancestor - a weird grain called teosinte. It has just a few kernels on each stalk, and they're too rock hard to eat or to grind into flour.

PIPERNO: It'd be hard to make a tortilla out of it.

COLE: But Piperno says teosinte has one property that might make up for the whole no-tortillas thing.

(SOUNDBITE OF POPPING CORN)

PIPERNO: All early corns were popcorns. They were around for millennia before these other forms of corn.

COLE: After a couple thousand years, the Mesoamericans managed to cultivate varieties of corn that were good for flour, but they still ate popcorn. The Aztec language even has a word for the sound of many kernels popping at once: totopoca.

After the Spanish invaded, popcorn spread around the world. And soon, people began to figure out how popcorn actually works. It turns out that rock-hard kernel - the thing that makes teosinte and popcorn impossible to eat raw - is the key.

DAVID JACKSON: It acts as a pressure cooker.

COLE: That's David Jackson, a food scientist at the University of Nebraska. He says the durable kernel keeps water and starch sealed inside. When a kernel is heated, the starch liquefies. The pressure builds and then totopoca.

(SOUNDBITE OF POPPING CORN)

JACKSON: The pressure cooker essentially fails and it explodes outward into a popped kernel.

COLE: The seed coat breaks and the liquefied starch froths outward, cooling and solidifying in a fraction of a second. If you look at a popped kernel under a microscope, you can actually see the bubbles that were formed by the expanding steam. That's why popcorn is so light and fluffy, it's made of bubbles - and there's something to think about next time you're stuck watching a bad movie.

(SOUNDBITE OF POPPING CORN)

COLE: Adam Cole, NPR News.

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