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DAVID GREENE, HOST:

Here on MORNING EDITION we have turned our attention to the science behind summer activities. After all, the simplest questions can really lead to the most interesting scientific explanations. NPR science correspondent Joe Palca has agreed to help us sort through some of the mysteries that pop up on those long, lazy days of summer. And today, the epic quest for campers everywhere - how do you get that perfect roasted marshmallow?

JOE PALCA, BYLINE: These are really fine marshmallows, and we have until September 2012 to use them according to the package.

GREENE: That was Joe's voice, and he convinced Dan Madryzkowski from the National Institute of Standards and Technology to assist him in his scientific quest. They built a campfire and Joe took some pointers from the expert.

PALCA: And I'm going to hold it over this fire.

DAN MADRYZKOWSKI: You want to stay a little to the side of the flames, and also over an area where you see glowing coals.

PALCA: There are two kinds of heat coming off the fire.

MADRYZKOWSKI: If you're above the fire, you're in the convective flow.

PALCA: That's the hot gasses coming off the fire's flame. Though its flames are dancing all over the place, it's hard to stay in the sweet spot, and you can catch the marshmallow on fire if you're not careful.

MADRYZKOWSKI: But if you want to get it nice and toasted, you typically rely on radiant energy.

PALCA: That's heat coming off the glowing coals. Those coals give a more even, more consistent heat.

MADRYZKOWSKI: You could put it down a little lower, like probably right there.

PALCA: Wait, wait, we have disaster. It's caught fire. Dan explained why the marshmallow caught fire.

MADRYZKOWSKI: Because that marshmallow is a piece of fuel. Marshmallows are made predominately of sugar, and what's in the sugar?

PALCA: Sugar is made of carbon and hydrogen and oxygen, and as Dan Madryzkowski will tell you, carbon and hydrogen and oxygen are...

MADRYZKOWSKI: All the things that we need to make a nice piece of fuel.

PALCA: So now we understand why marshmallows can burn when you heat them over a fire. But if you manage not to torch them, they get a little brown and they puff up as you hold them over the coals. Why do they puff up? Well, there are pockets of air inside the marshmallow which is why it's so nice and squishy. As the marshmallow heats up, those pockets of air expand, and if you're a really good roaster, you get a delicate soufflé-like quality. There's a bit of summer science for you. Joe Palca, NPR News, Washington.

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