Concrete is so much more than just a sidewalk. We use it to build playgrounds and skateparks and even musical instruments! But how does it go from a powder to a sludge to the strong building material that we use all over the world? And can we come up with new recipes that are better for the environment?

To celebrate this superlative substance, Molly and cohost Mark are joined by special guest Concreature, a being from the concrete dimension! Tune in for a dose of history, some magnificent moss, and some hot, hot science.

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MARC: You're listening to Brains On, where we're serious about being curious.

CHARLIE: Brains On is supported in part by a grant from the National Science Foundation.

MOLLY BLOOM: Ah, what a perfect day to be outside, staring at concrete. Right, Marc?

MARC: Totally. The sky's clear, the sun is shining, and the sidewalk is so flat and so gray. Couldn't be better.

MOLLY BLOOM: I wonder if our listeners would love learning about concrete like we love learning about concrete.

MARC: I know, right? It's everywhere, but nobody ever talks about it. That building?

MOLLY BLOOM: Concrete.

MARC: That wall?

MOLLY BLOOM: Concrete.

MARC: That plant pot?

MOLLY BLOOM: Concrete.

MARC: This curb outside Brains On headquarters that is a perfect, calming shade of gray?

MOLLY BLOOM: More concrete.

MARC: If you look close, there are even little sparkly bits in there. And wait-- is that a face?

MOLLY BLOOM: Where? Let me-- oh, yeah. The sidewalk has a face. And it's growing.

CONCREATURE: Well, slap me flat and call me a sidewalk. You two are making this concreature blush.

MARC: What in the whole wide world of whacked-out weirdness is that?

MOLLY BLOOM: It's a giant talking blob of concrete?

CONCREATURE: I'm Concreature. I live in an alternate dimension where everything is concrete, including me. Here's my business card.

MOLLY BLOOM: Thank you. Oh, yup, that's just a square of wet paper covered in concrete.

MARC: From another dimension? A concrete dimension?

CONCREATURE: Yup. I heard you saying such nice things about concrete on my inter-dimensional podcast app, and thought I'd beam over to say, hi.

MARC: Well, it's not the weirdest thing to ever happen at Brains On headquarters.

MOLLY BLOOM: Not even the weirdest thing this week, honestly. That's starfish roller disco last night was much weirder. OK, Concreature, how about you come to the studio and you can help with the show?

CONCREATURE: Yay! I even brought my own microphone. It's also made of concrete.

MOLLY BLOOM: You are listening to Brains On from APM Studios. I'm Molly Bloom, and my co-host today is Marc from Wellington, New Zealand. Hi, Marc.

MARC: Hi, Molly.

MOLLY BLOOM: I am so glad you could help me host the show today. You are someone who lives in New Zealand, but you also spend time in the States, right?

MARC: Mm-hmm.

MOLLY BLOOM: Would you say there's any difference in the concrete in New Zealand versus the concrete in the United States? Or is it the same?

MARC: It's the same.

MOLLY BLOOM: Concrete's the same wherever you go?

MARC: I think.

MOLLY BLOOM: Yeah, it's kind of like looking at the moon. No matter where you go, concrete's the same-- so sturdy, reliable. If you could make anything out of concrete, what would it be?

MARC: Big ramps from my bike.

MOLLY BLOOM: Ah, nice. Do you like to do tricks on your bike?

MARC: Yeah.

MOLLY BLOOM: What kind of tricks?

MARC: Um, I'm not very good at tricks right now. But I can go in the air, like, off a jump.

MOLLY BLOOM: That is way more than I can do. Why do you think concrete is so common?

MARC: It's just really, really, really strong.

MOLLY BLOOM: Yeah, it's pretty amazing when you think about how they do use it for things like bridges, and tunnels, and sidewalks, and buildings, and all these-- like, just a really huge diversity of things. Well, to help us with today's episode, we're joined by-- is it Mr. Concreature?

CONCREATURE: Please. Mr. Concreature was my father. You can just call me, Concreature.

MARC: And you're a visitor from a dimension that is all concrete?

CONCREATURE: Pretty much. Yeah. And boy, are you all lucky that you have this stuff here, too. I've seen dimensions with zero concrete, and they are not fun. Here, I can show you. Let me open a picture portal. Look at the chaos of a concrete-less world.

MARC: Oh, no! There aren't any roads. Cars are permanently stuck in the mud.

MOLLY BLOOM: There aren't any big bridges, so nobody can cross wide rivers or bays!

MARC: There aren't sidewalks either. Pedestrian panic!


WOMAN: How is my child supposed to walk to school?

WOMAN 2: And skaters have no skateparks. They're trying to skate on grass, and it's going really badly.

SKATER: Oh, man. You really ate grass back there. Total wipeout!

SKATER 2: Ya, bro. And I got grass stains all over my brand-new ripped jeans again! Ugh!

MARC: Wow. OK. Now, I have so many more questions about concrete.

MOLLY BLOOM: And come to think of it, a lot of our listener pals have sent in questions, too.

GRACE: My name is Grace from Port Washington, New York. And my question is, how does concrete work?

CAM: My name is [? Cam, ?] and I'm from New Mexico. My question is, how does concrete get hardened?

JOHN: Hi, my name is John. I am from Queens, New York. And my question is, how are cement and concrete made?

MARC: So Concreature, are cement and concrete the same thing?

CONCREATURE: Actually, they're different. Cement is used to make concrete. It's an ingredient, like when I add a little cement to my gravel smoothie in the morning. Good for the digestion. It's like that stuff you eat on Earth. I think it's called, uh, breab? Aborb? Brad?


CONCREATURE: That's the one. It's made of different ingredients, right? Like powdery stuff called flour, and watery stuff called water.

MARC: Right.

CONCREATURE: That mixture of flour and water is like cement. Now, you could leave the flour and the water like that, bake it, and it would just be plain bread. But maybe you want to add things to your bread to make it tastier and more filling.

MOLLY BLOOM: Oh, like seeds or oats or raisins or chocolate chips or something?

CONCREATURE: Yes, exactly. That's like concrete. Concrete is like the seed oat chocolate chip raisin bread. The cement is like the flour and water that holds all the bits together. But the whole loaf with the extra bits is concrete.

MARC: So concrete is a mix of cement and other stuff. The cement binds everything together. They're different, but you can't have concrete without cement.


MOLLY BLOOM: OK, so how do you make cement?

CONCREATURE: You humans have been making cement for a very long time. The first people to really nail it were the ancient Romans more than 2,000 years ago.

MARC: Whoa. Cool!

CONCREATURE: And the recipe has only changed a little bit since then. To start, you have to use a certain type of stone. I think you call it limestone. In my dimension, we call it, Timothy.

But anyway, you cook the limestone in a hot oven-- I mean, hot, around 2600 degrees Fahrenheit. So hot.

MOLLY BLOOM: Whoa. That's, like, three pizza ovens combined.

CONCREATURE: It's hot enough to actually change the chemistry of the limestone. That means it kind of becomes something new after you bake it, and it gets rid of all the water molecules inside the rocks. Then once the rocks cool down, you crush them into powder.

MOLLY BLOOM: And that's powdered cement? I've seen that at the hardware store down the street where I go to get new doorknobs for my doorknob collection.

MARC: You have a doorknob collection? I have a doorknob collection.


MARC: Way! So then, you add water to the powdered cement?

MOLLY BLOOM: And you add extra stuff like the oats and seeds in bread to make it concrete?

CONCREATURE: Yeppers. Except, we don't add oats and seeds to cement. We usually add sand or gravel. And get this. When the water touches the cement, it actually combines with it.

And when it does, it creates a chemical reaction that actually gives off heat. That's called an exothermic reaction. It means that as cement hardens, it gets hot.


CONCREATURE: Very so hot. And then, the concrete cools and hardens, and ta-da! A sidewalk, a building, a concrizza. That's a concrete pizza, my favorite lunch. I guess that one's only in my universe, but you get the idea.

MOLLY BLOOM: Oh, so that's why concrete is so popular as a building material. It starts off as liquid goop, and you can pour it into shapes?


MARC: And I bet bags of powdered cement are easier to move around than big blocks of rock or something.

CONCREATURE: Yep. But there's more. You can pour concrete into lots of shapes to make a giant bowl for an underground pool, make strong pillars, or sloped ramps for wheelchairs. You can make walls that you stand up to make houses.

You can add metal bars to wet concrete to make it extra strong, so it can be the walls of a skyscraper. And concrete is waterproof, weatherproof, fireproof, and lasts a super long time. It could do so many things that no other material can. That's why it's so popular here on Earth.

MARC: Who knew you could make so much stuff out of concrete?

CONCREATURE: Well, then, hold onto your butts, because there's more. Did you know that a human in your world even made a guitar out of concrete? Here's what it sounds like.


Pretty cool, huh?

MOLLY BLOOM: Very cool. Speaking of nifty noises, Marc, it's time for the--

SPEAKER: Mystery Sound.

MOLLY BLOOM: You ready to guess the mystery sound?

MARC: Yeah.

MOLLY BLOOM: Here it is.


MARC: Is that like some sort of slap-a-phone or something?

MOLLY BLOOM: What's a slap-a-phone?

MARC: I think it's like an instrument that I made up in my mind.


MOLLY BLOOM: I like that. What-- tel me what that instrument looks like.

MARC: Tubes. And then, you have foam things, and you slap the tubes.

MOLLY BLOOM: Oh, yeah.

MARC: And the tubes are different--

MOLLY BLOOM: Oh, I don't think you made that up. I think that's a real thing. I've seen that before. I don't-- I don't know if it's called a slap-a-phone, but if it's not, it should be. Because that's a really good description. Do you want to hear it again?

MARC: Yeah.



MARC: It's a really weird noise. It kind of gives me the impression of, like, electrical things, but they sound too deep. [CHUCKLES]

MOLLY BLOOM: Yeah. And it does sound kind of musical. Like, there's, like, a rhythm happening.

MARC: A drum underwater.

MOLLY BLOOM: Ah, I like it. All right. So we'll keep thinking it over, and we'll have another chance to guess after the credits.

We love hearing from you. Your mystery sounds bring us joy, and your drawings make our days. And your questions are the inspiration for every single episode.

MARC: Send them to us at

MOLLY BLOOM: That's where we got this one.

ELI: Hi, I'm Eli.

SIMONE: And I'm Simone. We're from San Francisco, California.

ELI: And our question is, do fish get thirsty?

MARC: You can find an answer on the Moment of Um podcast.

MOLLY BLOOM: It's a dose of facts and fun every weekday. Find it wherever you listen to this show.

MARC: Just search for Moment of Um.

MOLLY BLOOM: And keep listening!

You're listening to Brains On. I'm Molly.

MARC: I'm Marc. And this is our special other-dimension guest, Concreature.


MOLLY BLOOM: So far, we've learned that concrete is everywhere-- your grandma's patio, your playground.

MARC: Your tall buildings, your sewer pipes.

CONCREATURE: Your socks and underwear.


No? That's just in my dimension? OK, but you're missing out.

MARC: Humans have been using concrete for thousands of years. It's all over the world.

MOLLY BLOOM: Here's the thing about concrete, though. It rarely blends in with nature, but nature sometimes blends in with it.

MARC: Yes. Sometimes older concrete structures get all green and mossy, which is super interesting, because you wouldn't think that concrete would be a very pleasant place to grow. But moss loves to grow on concrete, and it does that in a really interesting way.

MOLLY BLOOM: We had a question from a listener about that.

LILA: My name is Lila.

FINN: And my name is Finn.

LILA: We're from Stillwater, Minnesota.

FINN: Our question is, how does moss stick to concrete?

LILA: And how does the roots get inside?

DANIEL STANTON: It's a really rough place to grow. There's no soil. There's only water when it rains.

MARC: That's Daniel Stanton.

MOLLY BLOOM: He studies moss at the University of Minnesota.

MARC: And if you think concrete is tough well, wait until you meet moss.

DANIEL STANTON: So moss is a plant, but instead of getting bigger and bigger, they got better and better at handling really harsh places. If you look at it up close with a magnifying glass or under a microscope, you'll see that it does look like a little glass. It's got a tiny little stem and little leaves.

MOLLY BLOOM: And even though moss didn't evolve to grow on concrete, it did learn to grow on rocks and other places that don't have soil or a lot of water.

MARC: That's why concrete and moss are a great match.

SPEAKER: Moss, darling?

SPEAKER 2: Yes, concrete-y, my sweetie?

SPEAKER: There's no one I'd rather share this little patch of the world with than you. I thank my lucky stars your spores landed here all those years ago. Happy anniversary.

SPEAKER 2: Oh, concrete-heart, there's no surface I'd rather grow on.


MOLLY BLOOM: Even though moss doesn't have roots, it can soak up what it needs from dust, rainwater, and sunlight.

MARC: So part of why moss does well on concrete is because it's already adapted to live in some of the harshest places on the planet.

DANIEL STANTON: Some of the moss species that we see most often in the city are the exact same moss species that are found in Antarctica. And I've seen them in really high mountains, like at the edge of glaciers in the Andes. And a lot of those same adaptations allow them to be growing on the edges of Antarctica or the world's highest mountains.

MOLLY BLOOM: But moss isn't the only living thing that's learned to thrive in a concrete world. Daniel says there's a special type of bird that turned out to be perfectly suited to a city full of cement-- pigeons.

DANIEL STANTON: In the wild, pigeons are only on some cliffs. And yet, because they're used to living on cliffs, they do great in cities. The mosses that we find on concrete are doing the same kind of thing.

There's this new space that's opened up for them that has a whole lot of open surface, that nothing else is able to move in on.

SONG: Ba, ba, ba, ba, ba, ba, ba, ba, ba, brains on.

MOLLY BLOOM: Even though pigeons and moss have learned to live with concrete, it can also cause problems for the natural world.

MARC: Yeah. One of the problems is that it adds to climate change.

MOLLY BLOOM: You may have heard about climate change. It's caused by carbon dioxide and other greenhouse gases in the atmosphere. We release carbon dioxide just by breathing, but there are other things we do that produce carbon dioxide, like using gasoline. Our planet does need some carbon dioxide.

MARC: Carbon dioxide is like a big, invisible blanket in the sky, because it traps heat and makes things warm.

MOLLY BLOOM: But when there's too much carbon dioxide in the sky, it traps a lot of heat, and that can warm the planet in dangerous ways.

CONCREATURE: Wait. So how does concrete fit into this? And is there anything we could do about it? I hate the idea that concrete is anything but a pure, great joy for the creatures of this planet.

MOLLY BLOOM: Well, we asked our pal, Lydia [? Morel, ?] to look into it, and she should be here now.


LYDIA: Hi, Molly. Hi, Marc. Hi, large talking blob of wet pavement.

CONCREATURE: Oh, hi, medium-sized collection of organs and bones. And just so you know, I am so much more than just pavement. I am Concreature.

LYDIA: Cool! I'm Lydia. Pleased to meet you.

MARC: So Lydia, you're here to tell us how concrete adds to climate change?

LYDIA: Yeah. Like you talked about earlier, when we make concrete, we have to heat limestone up to about 2,600 degrees Fahrenheit, which is so--


LYDIA: I was going to say so many degrees, but yeah, sure. But to heat up that limestone, we need to burn a lot of energy. And a lot of the time, that means burning gas or coal or other things called fossil fuels.

These are great at powering our power plants, making electricity, and heating up ovens. But when we burn them, they also give off a lot of that gas, carbon dioxide.

MOLLY BLOOM: Right. And CO2 traps heat in the atmosphere and warms up our planet.

MARC: So hot, but like, in a bad way.

LYDIA: Yeah. And when you heat up limestone, it also gives off CO2 as a gas. So that's causing even more climate change when we make concrete. And we make a lot of concrete.

CONCREATURE: I'm from a dimension made of nothing but concrete. So when you say a lot, what are you talking about exactly?

LYDIA: On this planet, we make about 4.4 billion tons of concrete each year. That's the same weight as 2 billion cars.

CONCREATURE: Oh. That is a lot. Touché.

LYDIA: And it makes a lot of carbon dioxide gas. In fact, if the concrete industry was a nation, it would be in third place for putting out the most CO2 behind China and the United States.

MARC: So should we stop making concrete?

MOLLY BLOOM: We saw the concrete-less world in the portal. Without concrete, roads and ramps--

MARC: Cars were in chaos. Skateboarders couldn't hang loose.

LYDIA: You're right. It'd be hard for us to quit concrete, cold turkey. But scientists do have some ideas to make it less bad for the planet. One person working on it is Dr. Josephine Chung, who studies how to make concrete greener.

CONCREATURE: Green, as in the color of the moss in my armpits?


LYDIA: Wow. You have the prettiest armpits I have ever seen-- [SNIFFS] --with such a nice, earthy smell! But no, not green like armpit moss. Green like better for the planet.

Josephine says one idea is to use different ingredients, instead of just limestone. So you might add in stuff that's left over from making steel or burning coal.

JOSEPHINE: And also, clays, which-- there's plenty of place on this Earth. And the use and [? substitute ?] of the clays can also help in making a good cement materials.

MOLLY BLOOM: The reason these materials are better for the planet is because they don't have to be heated up as much as limestone, which means you don't burn as much fossil fuel powering that oven.

MARC: So using different ingredients in concrete can cut down how much energy we need to make it? Got it.

LYDIA: Yep. And other researchers are working on ways of grabbing CO2 that's given off by factories and pumping it into the concrete mixtures that's being mixed, which can even make the concrete stronger.

MARC: Wow. So there are a lot of ways to make concrete greener, and none of them involve mossy armpits.

LYDIA: Yeah. But none of these ideas stop all the CO2 from going up into the air. They just cut down on how much. So it's something we're going to have to keep working on.

CONCREATURE: Whew! For a minute there, I thought I was steamrollered. But this gives me some great ideas for new concrete recipes. I'd better get back home and get cooking with all these new flavors. Clay concrete? Mm-mm-mm.

MOLLY BLOOM: Sounds-- delicious? Well, thanks for joining us, Concreature, and thanks for the scoop, Lydia.

LYDIA: Sure thing.

MARC: Bye!

MOLLY BLOOM: Concrete is a very strong, very useful material.

MARC: It's made by mixing cement, water, and other materials into a sludge.

MOLLY BLOOM: You can shape that sludge into all kinds of things.

MARC: Some living things have even made concrete their home, like moss and pigeons.

MOLLY BLOOM: But when we make concrete, it adds to climate change. So we're working on new recipes that are better for the planet.

MARC: That's it for this episode.

MOLLY BLOOM: Brains On is produced by Molly Bloom, Rosie DuPont, Anna Goldfield, Ruby Guthrie, Nico Gonzalez Wisler, Aron Woldeslassie, and Marc Sanchez.

MARC: Our editors are Shahla Farzan and Sanden Totten.

MOLLY BLOOM: We had engineering help from Peter Molteno, and this episode was sound design by Rachel Brees. Our executive producer is Beth Pearlman, and the APM Studios executives in charge are Chandra Kavati, Joanne Griffith, and Alex Shafford. Our intern this fall has been the wonderful Lydia [? Morel. ?]

Special thanks to Ruth Fisher, Adam Smith, Matthew Adams, and voice actor extraordinaire, Brant Miller.

MARC: Brains On is a nonprofit public radio program.

MOLLY BLOOM: All right, Marc. We're going to go back to that very mysterious mystery sound again. Are your ears ready?

MARC: Mm-hmm.

MOLLY BLOOM: OK. Here it is.


MARC: Now, I kind of think it's like something falling down a hill or something.

MOLLY BLOOM: Oh. I can hear that. Yeah, like, bump, bump, bump, like falling down the stairs or hitting things on the way down?

MARC: Yeah.

MOLLY BLOOM: I can totally-- so what would be making that noise, do you think?

MARC: Maybe like a weighted mattress?

MOLLY BLOOM: Ooh, interesting.

MARC: Ooh, a weighted mattress getting hit by hammers.


MOLLY BLOOM: I love it. And it sounds like they're making music to me. So maybe that's a different instrument that can go with the slap-a-phone. It's called the-- I don't know, what would you call that instrument?

MARC: The hammerphone.

MOLLY BLOOM: The hammerphone. Perfect. All right, are you ready to hear the answer?

MARC: Yeah

MOLLY BLOOM: Here's the answer.

LUCA: Hi. My name is Luca from Wylie, Texas. The sound you just heard was me popping rubber bands.


MOLLY BLOOM: Rubber band. Gotcha. Yeah, OK, so yeah. It's almost like he had a guitar made from rubber bands or something. Because it sounded so musical.

MARC: Or maybe like he strung the rubber bands up, and then took a knife and cut them.

MOLLY BLOOM: Oh, and that was, like, kind of the sound of them. Oh, yeah, 'cause he said popping rubber bands, right? Is that he said, Marc?

MARC: Yeah, I think so.

MOLLY BLOOM: OK. I think we can have a whole band. We can have the slap-a-phone, we can have the hammerphone, and then the rubber-band-pop-a-phone. It'll be beautiful. I can't wait.


Now, it's time for the brain's honor roll. These are the incredible kids who keep this show going with their questions, ideas, mystery sounds, drawings, and high-fives.


We'll be back soon with more answers to your questions. Thanks for listening.

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