Is time travel possible?

PENELOPE: You're listening to Brains On, where we're serious about being curious. Brains On is supported in part by a grant from the National Science Foundation.

SANDEN TOTTEN: OK, so picture a washing machine. But once you went inside, you find a big chamber with a beaded curtain for a door. Oh, and then through here, you'd see this wonderful purple lamp, very groovy, and probably the whole thing should smell like buttered popcorn. Yeah, nice touch. OK, from there--

MOLLY BLOOM: Hey you guys, what are you drawing?

SANDEN TOTTEN: We're brainstorming cool time travel devices.

MOLLY BLOOM: Oh cool, so like what you'd use to travel through time.

SANDEN TOTTEN: Exactly, it's for the next episode. This one's getting very complicated. So we needed to diagram it.

MOLLY BLOOM: Mmm, I see, what other devices have you thought about?

SANDEN TOTTEN: Oh, my favorite was the pogo stick. Why stroll into a different era when you can bounce.

MOLLY BLOOM: So true. I'd have to go with a piece of cake. Yeah, because that way I could eat the piece of cake, and then go back in time to before I ate it, and eat it again.

SANDEN TOTTEN: Cake, Oh come on, that's not a vehicle. Man, this makes no sense. Well this is time travel, none of it makes any sense if you think about it.

MOLLY BLOOM: And I'd get to eat cake twice, double win.

[MUSIC PLAYING]

MOLLY BLOOM: This is Brains On from American Public Media. I'm Molly Bloom. And joining me today for an adventure through space and time is Penelope from Williamsburg, Virginia. How you, Penelope.

PENELOPE: Hi Molly.

MOLLY BLOOM: Today's show was inspired by a question that you asked us?

PENELOPE: Is time travel possible?

MOLLY BLOOM: So what got you thinking about time travel? Why did you send us the question?

PENELOPE: Oh, because I really wanted to see the past, and I thought of like, hey, what about time travel? What would you do?

MOLLY BLOOM: Is there a specific place, or I should say, is there a specific time and place you want to go to in the past?

PENELOPE: I wanted to see that age of the caveman, so I can just bring a box of matches with me to teach them how to make fire.

MOLLY BLOOM: Well we asked our listeners when they would like to travel to, and they had a lot of thoughts.

ATTICUS: I would time travel to 2024, because that's when humans might launch to Mars.

HUGO: Far into the future, because I would want to see how new gaming devices work.

LAURA: To see how I look like, and what's my future dog.

MILA: To see if dogs still exist, or if they were replaced by robots.

MOLLY BLOOM: That was Atticus, Hugo, Laura, and Mila.

PENELOPE: We'll hear more of your time travel answers throughout the show.

MOLLY BLOOM: But first, let's tackle your most excellent question, Penelope. Is time travel even possible?

SANDEN TOTTEN: Greetings people of the 21st century.

PENELOPE: Whoa, who turned out the lights?

SANDEN TOTTEN: Prepare to be amazed as I present you with a hot steaming plate of the amazing, served with a side salad of the impossible.

MOLLY BLOOM: That voice sounds super Sandeny.

SANDEN TOTTEN: Presenting now a device so revolutionary it makes sliced bread look like chopped liver. A device so fantastical, even Bigfoot said, whoa that's hard to believe. A device that will change the world forever, and all time past, present, and future. Cue the lights. Feast your eyes on my time machine. Ha ha.

MOLLY BLOOM: Sanden, that's just an old tape recorder that you wrote time machine on in puffy paint.

SANDEN TOTTEN: Ah, but it is so much more.

PENELOPE: By that, do you mean it's also covered in glitter?

SANDEN TOTTEN: Yeah, the glitter thing. OK look, it's the best I could come up with on short notice. But Molly and Penelope, with my patented device I guarantee that you will visit the past by the time I leave this studio. In fact, thanks to my device, there are people from the future with us this very second.

[THUNDER]

PENELOPE: I don't see them. Oh, just you wait. But first, let's tackle your question. Is time travel possible? Well let's talk about time itself. In the olden days, many people thought of time as like an arrow. Once the arrow is shot from a bow, it flies forward, and then goes on, and on in one direction forever.

MOLLY BLOOM: Yeah, I'd say that's pretty much how time feels.

PENELOPE: Very arrow-like.

SANDEN TOTTEN: Right, but it turns out time might not be an arrow, it might be a river. It flows in one direction, sure, but sometimes it speeds up, and sometimes it slows down. It whines and meanders around large objects. It can splash you, get your pants wet.

OK, time doesn't actually do that last one. But this idea of time as river comes to us thanks to a famous scientist named Albert Einstein. Penelope, have you heard of him?

PENELOPE: For a Christmas I've actually wanted a huge poster of him just hanging in my room. Amazing I just really like him, because he discovered some pretty important things about our universe, which also inspires everyone else, including me to do stuff like that.

MOLLY BLOOM: Yes, that's so cool. He did what's called thought experiments. So he was always daydreaming and thinking. And he looked at the world around him, and he wanted to know how the universe worked. And he did it all just by thinking, and doing math, and pen and paper. And came up with these big ideas that changed the way we saw the universe.

SANDEN TOTTEN: Right, exactly. And Einstein equals cool guy. According to Einstein's theories, time is actually relative, meaning it changes depending on a few things. For instance, the faster something moves, the slower it experiences time. This is called time dilation.

PENELOPE: So if I get up and run around really fast, like this, whaaa, then time was moving slower for me than it was for Molly?

SANDEN TOTTEN: Yes. But sadly, at that speed the difference is so tiny and so imperceptible that there's no way anyone would notice.

PENELOPE: What if I ran faster?

SANDEN TOTTEN: Well you'd have to go a lot faster. For example, a few years ago two twin astronauts did an experiment. One, named Scott Kelly, lived on board the International Space Station for almost a year. His twin brother, Mark Kelly lived on Earth. Now the Space Station goes by really fast, like four miles a second. So that means time on board is slightly slower than here on Earth.

So when Scott came back down he had actually aged less than his brother Mark had, which is time travel, ha, ha.

PENELOPE: Wow, so now he's younger than his twin?

SANDEN TOTTEN: Yep, but only by 13 milliseconds.

MOLLY BLOOM: Jeesh, that's not much.

SANDEN TOTTEN: No it is not. But given this principle of relativity, you could theoretically build a ship that goes super duper, extra pooper fast, almost a speed of light. Zip around the Earth for, I don't know, 15 years, and then when you land, it would be 50 years later on Earth, which is time travel into the future, ha, ha.

MOLLY BLOOM: Wow, living on a spaceship for 15 years sounds super dull, but I guess you would get to see the far future in your lifetime.

PENELOPE: Yeah, flying cars, shiny clothes, ice cream lasers, and cyborg kittens. I'm just spitballing here, but I heard that'd be pretty awesome.

SANDEN TOTTEN: Whoa, I like where your head's at. The other way to slow your experience of time is with gravity. So the more gravity something has, the slower time goes. So if you could somehow go hang out on a super large planet, which would have a ton of gravity and you stayed there for a few years and somehow not getting smooshed by all that gravity, then you somehow came back to Earth, you would have aged less than everyone else. How you would do all that, I have no idea.

PENELOPE: I like the spaceship option better.

SANDEN TOTTEN: But the catch with both, is that it's a one-way ticket. You can travel into the future, but you can never go back.

MOLLY BLOOM: That suddenly seems less exciting. I would not want to see the ice cream lasers and cyborg kittens if I couldn't tell my friends about them.

PENELOPE: So you're saying time travel backwards into the past is impossible?

SANDEN TOTTEN: Yes and no. I'll explain right after this.

MOLLY BLOOM: The following message was paid for by the past. Do you ever think to yourself, gee, things were better before when I had more friends, or more money, or more hair. Do you wish you could high five a pharaoh or play fetch with a T. rex.

Want to fix that embarrassing time you sat on a peanut butter jelly sandwich and you had a peanut butter jelly sandwich on your butt, and then someone said, hey look, it's peanut butter jelly sandwich but, and everyone laughed? We here at the past know how you feel, and we want to help you out. So come join us and explore what once was from ancient civilizations to last week's lunch, it's all here in the past. We also know where you left your keys.

[MUSIC PLAYING]

PENELOPE: You will go to the time of the dinosaurs. I will play with my triceratops pet.

MARA: 2, 1603, because that was in the years in England of the reign of good Queen Bess.

RUTHIE: I go meet the pioneer.

DAVID: I want to time travel.

LILLIAN: See the T. rex.

AKIVA: I just want to see how much dust it kicks up running.

[COUGHING]

MOLLY BLOOM: Thanks to Mara, Ruthie, David, Lillian, and Akiva for their answers. OK, where were we?

PENELOPE: Or when were we.

SANDEN TOTTEN: We Were Imagining traveling to the future with no way back.

MOLLY BLOOM: Oh right, that was a bummer.

PENELOPE: Please tell me a way to go backwards in time.

SANDEN TOTTEN: OK, I do. Here's the thing. Given the laws of the universe as we know them, it seems highly unlikely that time travel to the past is possible. But humans are clever, and we have some interesting ideas.

MOLLY BLOOM: OK, we are listening.

SANDEN TOTTEN: So one way to travel into the past would be to go faster than the speed of light. But most scientists think that's just not possible, given the laws of physics as we know them. But there is a design for something called an Alcubierre warp drive. This is a machine that doesn't go faster than light, but instead it warps space time itself. Space time is the name we give for a model of the universe, where you map both height, depth, width, and the dimension of time.

PENELOPE: Time, the fourth dimension.

SANDEN TOTTEN: Exactly. So this warp drive theoretically could compress and smoosh up space time in front of it, and expand and spread out space time behind it, it would be like, instead of driving down a long road, you just folded the road underneath you until you got where you wanted to go.

The math behind this idea is super complicated, and it really only works if we could find things that might not exist in the universe, like some form of negative energy. Then again, [AUDIO OUT] I have plenty of negative energy before my coffee. You can have that. Not what I meant, Molly, but thank you.

Another possibility is our old friend, the wormhole.

PENELOPE: Oh yes, also known as the Einstein-Rosen Bridge. It's a theoretical structure that connects to disparate locations and spacetime through a special but unstable passage.

SANDEN TOTTEN: No, it's basically a magical tunnel that links to far off places in the universe. That's what I said. Anyway, these aren't mathematically possible, but there's so much we don't know, like do they actually exist?

Are they stable enough to travel through? Would going through one smooth shoe into a tiny ball of atoms. Is there food and drink service, or do you have to pack your own snacks.

Important questions. And there are a few other ideas too. But all of them have big questions that still need to be answered before we could even consider them real options.

PENELOPE: So that sounds like a note to the past? But you said you'd take us to the past before you left the studio.

SANDEN TOTTEN: I did, and I will. With my time-- Actually can I get that like suspenseful music back real quick.

[MUSIC PLAYING]

Perfect, OK. With my time machine--

[MUSIC PLAYING]

--behold as I turn back time. Hold on one second. OK, there we go. We're flying backwards into the past as we speak.

[MUSIC PLAYING]

Here we are.

[MUSIC PLAYING]

OK, Sanden, just add some more glitter and this time machine is ready. I'm such a genius. I'm such a genius, and I'm so handsome. Oh man it's really unfair how amazing-- Oops, a little too far back. I'm just going to hit this button.

[MUSIC PLAYING]

There we go. But Molly and Penelope, with my patented device, I guarantee that you will visit the past by the time I leave this studio. In fact, thanks to my device, there are people from the future with us this very second.

[THUNDER]

PENELOPE: I don't see them.

SANDEN TOTTEN: Oh just you wait. See, you were the visitors from the future all along. We just visited the past, thanks to my handy dandy time machine. Time travel, ha ha.

MOLLY BLOOM: That doesn't count.

SANDEN TOTTEN: Yeah I tried but. I'll leave you with this idea. My tape recorder is not a very robust experience of the past, I'll admit it, but it is the past. Today we have so much data about our present. We have entire maps of streets, and images of what cities look like from the ground level stored in our computers and in the cloud.

What if one day, 100 years in the future you could put on virtual reality goggles and walk around your neighborhood. And then lay over what you're seeing, images of what things looked like back in 2012, or 2015, or 2020 to someone in 2120. That would be like walking around in the past.

Yeah, you couldn't interact with anything. But it would be a time travel, like my tape recorder. And I think it would be pretty cool.

MOLLY BLOOM: That's an interesting idea.

PENELOPE: Yeah, but I still want the real thing. Oh well.

MOLLY BLOOM: Thanks for all this info, Sanden.

SANDEN TOTTEN: No problem. Now if you'll excuse me, I'm going to go invent ice cream lasers, because I have a pretty good feeling they're going to be huge in the future. Bye. Ba ba ba ba ba ba ba ba ba ba ba ba, brains on.

MOLLY BLOOM: Well let's dive into something that's possible in the here and now. It's time for the shh

CHILD: Mystery sound.

MOLLY BLOOM: Here it is.

[CLICKING SOUND]

OK, Penelope what is your guess?

PENELOPE: I think it sounds like someone like typing aggressively on a keyboard.

MOLLY BLOOM: Very good guess. We'll be back and give you another chance to guess in just a bit.

[MUSIC PLAYING]

Hey time travelers, we want to see how you would ride through space in time.

PENELOPE: Dream up your own time machine and draw it for us. Is it a time copter?

MOLLY BLOOM: Or time cake? Whatever it is we can't wait to see what you dream up.

PENELOPE: Send your drawings to us at brainstorm.org/contact.

MOLLY BLOOM: You can also send us your questions like Harland did.

HARLAND: Hi, I'm Harland from Stillwater, Minnesota, and I'm wondering how do clothes get dirty, even though you don't see it?

PENELOPE: We'll answer that--

MOLLY BLOOM: --and shout out the honor roll at the end of the show.

PENELOPE: So keep listening

[MUSIC PLAYING]

SANDEN TOTTEN: This ad is brought to you by The Future. Hey you know what's more exciting than today, tomorrow. Anything can happen tomorrow. That's right, the future is awesome. All your problems are solved. Food is pills now. Everything glows. There's probably jetpacks for dogs and robot butlers.

Some people want to visit the past, but that's like rereading the same book, or watching old TV shows. Come visit us at The Future and find out how it all turns out. Spoiler alert, it's awesome. The Future. As long as we solve climate change, it's probably great.

MOLLY BLOOM: This is Brian Zan from American Public Media. I'm Molly.

PENELOPE: And I'm Penelope.

MOLLY BLOOM: And this is the mystery sound again.

[CLICKING SOUND]

All right you thought, it was someone really aggressively typing before. Do you have any new thoughts?

PENELOPE: Or it's either like a whole bunch of fish flopping on the ground, because then it got that feeling. And maybe even one of those typewriters, because I've heard them before.

MOLLY BLOOM: There's a lot going on here. So maybe something animal-like, or maybe typing. It's a tricky one. Are you ready for the answer?

PENELOPE: Yes.

MOLLY BLOOM: Here's the answer.

JAMES: Hi, I'm James.

HENRY: And I'm Henry. And we're from--

JAMES AND HENRY: --Lackawanna, New York.

JAMES: That was the sound of a cicada flapping its wings in my hand.

MOLLY BLOOM: That fish flopping, you were almost there. It was an insect flapping.

HENRY: It sounds like crinkly candy wrappers. Our favorite things to do with cicadas is to watch them emerge from their shells.

JAMES: And collect their shells.

HENRY: The empty shells are brown, crispy, and very fragile. They are sometimes hard to find, because they have great camouflage. Cicadas have a special organ called a tymbal that makes sound. The tymbal has special ridges that make loud noises when the cicada bends in a certain way. Cicadas are very noisy.

[BUZZING]

MOLLY BLOOM: Even though, as Sanden told us, time travel is only, maybe theoretically possible.

PENELOPE: You can't stop thinking about the idea.

MOLLY BLOOM: It's in books and movies.

PENELOPE: Plays and songs.

MOLLY BLOOM: And listeners, I'll bet you have spent some time dreaming about it too.

PENELOPE: If you follow the rules of science fiction, you just find a time traveling device, flip a switch, and poof you are in the future or past. Congratulations.

MOLLY BLOOM: But if you follow the rules of science without the fiction, you run into what's known as a paradox.

PENELOPE: A paradox is when you have an idea that sounds good and true. But when you examine it logically, it just doesn't make sense.

MOLLY BLOOM: For example--

[MUSIC PLAYING]

For as long as you can remember your mom has been telling you stories about how she was the fastest runner on her high school track team. Nobody could beat her until she stubbed her big toe on a brick right before the final track meet.

PENELOPE: One day, on your way home from school, you discover a fire hydrant time travel portal.

[PING]

So you decide to go back to watch your mom run and save her big toe.

MOLLY BLOOM: You see her practicing, and she is fast. Nobody can catch her at all, Oh no, it looks like she's seen you, this can't be good. So you dive into the bushes to hide.

PENELOPE: In your scramble to conceal yourself, you accidentally knock a brick out and to the sidewalk, the very same brick that caused your mom to stub her toe.

[DRUM]

MOM: Ouch, my toe.

MOLLY BLOOM: This is called the predestination paradox. You're stuck in an endless loop of time and always the cause of the problem you're trying to fix.

PENELOPE: Or how about this.

[MUSIC PLAYING]

Let's say you are really into the musical, Hamilton.

MOLLY BLOOM: You know every word to every song, even the fast raps. You can do the dance moves, and you know the birthdays and biographies of all the cast members. You love this show so much, you can't imagine the world without it.

PENELOPE: And you also discover a time machine.

MOLLY BLOOM: So you decide to go back in time to see Lin-Manuel Miranda writing his masterpiece. But when you get there, he's like, I have no good ideas. I should just quit.

PENELOPE: And you're like, no, you can't quit, you have to write the greatest musical ever about Alexander Hamilton.

MOLLY BLOOM: And then he's like, a musical about Hamilton? That's a brilliant idea, I'll get started right away. Not throwing away my shot.

PENELOPE: But since you gave it to him, who actually wrote it?

MOLLY BLOOM: He would have never written it without your help, but you would have never told him about the idea if he hadn't written it. So the show would have never existed. Whoa.

PENELOPE: This is called the bootstrap paradox.

MOLLY BLOOM: One big theme in science fiction is that if you go back in time, you shouldn't mess with events, because that could change the future.

PENELOPE: Our actions have consequences, and those consequences could change the future you came from.

MOLLY BLOOM: Some people try to get around this paradox by opening up the multiverse.

PENELOPE: Every time you change an event, you open up an alternate path of reality, multiple universes.

MOLLY BLOOM: Any way you slice it though, time travel can be a little brain melting. Oh my goodness, just like ice cream lasers.

TY: Where I would like to time travel is ancient Greece, because I really like hoplites. Hoplites are Greek soldiers.

PRESHIA: I would see the megalodon because I know megalodons are bigger than great whites, and they are probably super, duper cool.

ELLIOT: I go to the future so I could buy a jet pack, then go back to present, then just fly to school in the jet pack every day.

MOLLY BLOOM: Those great time travel ideas were from Ty, Preshia, and Elliot. Thinking about some parts of time travel makes my brain feel like a pretzel.

PENELOPE: But that's never stopped us. We have a long history of thinking about time travel.

MOLLY BLOOM: We asked Lisa Yaszek to tell us about that. She studies science fiction at Georgia Tech.

PENELOPE: Hi Lisa.

LISA YASZEK: Hi, how are you?

PENELOPE: Good.

LISA YASZEK: Well thanks for having me on the show today.

PENELOPE: Why do you think people are so interested in time travel?

LISA YASZEK: I think we like time travel stories because time is one of the few things that we can't really control in this world. If you think about it, we know enough about biology that we can control some diseases, and make sure that we don't get sick, and make our lives easier.

We can build machines that cross vast spaces and create buildings that let us live in all kinds of dangerous climates. But we all have to travel the same way through time, at the same rate, and with the same endpoint.

PENELOPE: How old are time travel stories?

LISA YASZEK: So time travel stories are as old as civilization itself. The first time travel story that we know about is almost 2,500 years old, and that's the story of King Kakudmi from the Indian epic, the Mahabharata. And that was from 400 BCE, a really long time ago.

And in the United States we've been telling those kind of stories for a long time. We see the first story in the United States shows up in the early 1800s, and that's the story of Rip Van Winkle, who's a man who falls asleep for 100 years and then wakes up in the future.

PENELOPE: Wow, so it's kind of like time traveling, although I would just call it very long hibernating.

LISA YASZEK: Yeah, I like that. I agree with you. Let's call it time hibernation, how about that? Let's make up a new phrase.

PENELOPE: Time hibernation.

LISA YASZEK: I love it.

PENELOPE: How do you think history changed the way we think about time travel.

LISA YASZEK: Oh, that's a great question. I think what we see is, every time we get new ideas from scientists about time, and how time works, we start to see some changes in time travel stories. So early time travel stories were really based on magic, and then in the 1800s, all of a sudden you start to have time travel stories that are about clocks and vehicles.

And I think that happens because in the 1800s, all of a sudden people are really using a lot of Science and Technology to build new cities, and to create new products, and to build new vehicles, like steam ships, and steam trains, and even bicycles and the first cars, so that we could travel in all these new ways.

And I think that authors got really excited about that. And so they started imagining machines in their own stories. And we still see machines today. In Time Travel we've seen stories about police boxes, and phone booths, and cars, and even hot tubs that could take us through time.

When you get to the 20th century, and the first part of the 20th century, when Albert Einstein got really popular, people all of a sudden get interested in the science behind time travel, rather than just building the machines. They want to know well, how does time travel really work. And when I'm driving these machines around through time, what are the rules I have to follow. And you get a lot of stories about that.

And then in the 1960s and 70s, we see another shift in how scientists start to think about time. And they start drawing on the science of quantum physics to talk about something called the many worlds theory. And that's the idea that any time you would travel through time you would actually end up in a world that's a little different than the one that you came from.

And that's when you start to see stories, where people say, you know what, maybe it would be OK to go through and change time, because there's so many different worlds and so many different possibilities. Maybe that when we travel, it's OK if we engage them a little bit and things will turn out OK.

PENELOPE: Do you have time travel stories you'd recommend for kids? I do. My favorite time travel book when I was a kid was Madeline L'Engle's A Wrinkle in Time. And I still think that's a great story. And you know what? Even if you don't want to read a novel, you can watch the movie that came out a few years ago. It's really fun and really beautiful to look at. It's exciting to see time travel on the big screen.

MOLLY BLOOM: Well thanks Lisa for making time for us today, really appreciate it.

LISA YASZEK: Oh thanks so much for having me here. It's been a lot of fun.

PENELOPE: Bye Lisa.

LISA YASZEK: Bye Penelope.

[MUSIC PLAYING]

MOLLY BLOOM: This ad is paid for by the present.

Hello, greetings, salutations. We are the Present Incorporated. Sure the past is cool and the future has ice cream lasers, whatever those are. But here at the Present Inc, we want you to know it's OK to live in the now. You don't have to go about gallivanting off to the ancient ages, or zoom forward millions of years. We think right now is pretty cool.

Unlike those other times, I promised you everything. We promise you nothing more than being in the present moment. Here are some fun things to do in the present. Look around, be mindful, exist. So for good time travel and enjoy the classic feeling of just being alive right now, the present it's fine here.

[MUSIC PLAYING]

PENELOPE: One day it might be possible to time travel into the distant future.

MOLLY BLOOM: But based on what we know right now, time travel to the past seems highly unlikely.

PENELOPE: Time travel is fun to think about, but it's full of brain bursting paradoxes.

MOLLY BLOOM: But that hasn't stopped humans from writing stories about it for thousands of years. That's it for this episode of Brains On.

PENELOPE: Brains On is produced by Menaka Wilhelm, Molly Bloom, Marc Sanchez, and Sanden Totten.

MOLLY BLOOM: We had production help from David [INAUDIBLE], Cristina Lopez. Editing from Phyllis Fletcher and engineering help from Cameron Wiley. Special thanks to Spiros Mihalakis, Anna, Jessie and Miles Thomas, Duke [INAUDIBLE], Rosie DuPont, Luke Burbank, and Megan Tan.

You can support the show and help us keep making more episodes at brainson.org/fans.

PENELOPE: You can donate at the link, or check out our shop, where you can buy cool shirts and sweatshirts.

MOLLY BLOOM: And the Brains On book, that's brainson.org/fans.

PENELOPE: And now, before we go, it's time for the Moment of Um-- How do clothes get dirty even though you don't see it?

Well our clothes get dirty because clothes are made of fibers that attract soils, and they're attracting the soils from accidental stains and they're primarily attracting the soils that come off of our body. My name is Jennifer Honey, and I am a tight principle scientist, with a background in biomedical engineering. And I work on figuring out solutions and the science behind laundry.

So we do a lot of laundry. You may see a lot of laundry done in your home. We're doing even more laundry in our labs. We have hundreds of machines. And what we do is, a lot of times when we run our laundry in our laboratories, we are doing an analysis of the soils that are in there as well.

And so we sometimes collect samples of the dirt that comes off of that. 70% of dirt actually in your laundry and on your clothes is made up of invisible dirt and soils that can come from your body, like sweat, salt, skin cells, and sebum, which is a greasy body oil that ends up on our skin. The thing about these body soils coming off of our body, is they don't remain on our body. Our body tends to be in contact with fabrics every single day.

[MUSIC PLAYING]

MOLLY BLOOM: This list of names smells like a fresh breeze to me. It's the Brains honor roll. These are the listeners who sent us their questions, ideas, mystery sounds, drawings, and high fives.

[LISTING HONOR ROLL]

[MUSIC PLAYING]

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

PENELOPE: Thanks for listening.