98.6 degrees is hot. If it were that hot outside, you’d be sweaty and want to go jump in a nice cold pool. But the insides of our bodies are that hot all the time! And that’s because there’s a lot going on in there that creates heat.

In this episode, Molly and co-host Saathvik find out how our bodies keep our insides at a nice steady temperature (and why that’s harder to do when it’s hot outside!). And we’ll talk with Dr. Joe Alcock about why humans and lots of other animals get fevers. Plus, a new mystery sound guaranteed to make you sweat!

Featured expert: Dr. Joe Alcock is a Professor of Emergency Medicine at the University of New Mexico Health Sciences Center. Learn more about him on X.

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


CHILD 1: Brains On is supported in part by a grant from the National Science Foundation.

ROGER RIBBIT: Is this thing on?


[CLEARS THROAT] Greetings, Brains On listener. It is I, a frog.


I noticed this episode of Brains On is all about body temperature, specifically how the human body is always around 98 degrees. To that I say, seriously? The same? All the time? How boring.

That's like only dancing the Charleston all your life-- hacha chittity cha, chi-chi-chi cha-- and never attempting tap.


Tippity, tippity, tippity, tippity, tap. Hey. Or only singing operatic style-- ladi dadi daaa-- and never scatting-- bibidy, dapidy, bibidy, dapidy, bibidy, dapidy, ba. As a frog, I don't keep my body at one single temperature. I am the temperature of my surroundings.

Sometimes I'm hot. Sometimes I'm cool. Same with other amphibians, reptiles, insects and fish. And have you noticed humans are always eating, because they need so much energy to keep their bodies warm?

(IN A WHIMPERING VOICE) Excuse me. I use my thumbs to open this banana and use these giant teeth to mash them up into spaghetti. Num, num, num, num, num, num, num.

Do you see me doing that? A flick of my tongue, and there's a delicious fly in my belly. [SLURPS]

Once again, it goes to show that humans are the inferior animals, and frogs, we've got the upper leg.


And another thing.




I hear one of the humans coming. Excuse me.


MAN 1: Oh, there you are. Roger Ribbit. How did you get out of your tank and into the studio? Back under the heat lamp you go.



MOLLY BLOOM: You're listening to Brains On from APM Studios. I'm Molly Bloom, and my cohost today is SAATHVIK from Danville, California. Hi, Saathvik.

SAATHVIK: Hey, Molly.

MOLLY BLOOM: Now, here's a little behind-the-scenes into how Brains On gets made. We get hundreds of questions from listeners every month. We read them all. It's so fun. And then we choose a number of these questions to answer, and that's how we pick our episode topics.

Since we get so many questions, sometimes it takes a while to get to the episodes we want to do, and this one took longer than most. Saathvik wrote to us six years ago when he was eight with an excellent question.

SAATHVIK: Yeah, I was curious about human body temperature. I wanted to know if the average, normal temperature inside the human body is 98.6 degrees Fahrenheit, then why don't we feel hot the same way we do when it's 98 degrees outside?

MOLLY BLOOM: It is such a good question. If 98 degrees is normal inside our bodies, why does it feel so hot when it's 98 degrees outside? It's been on our list of possible topics for a long time, but we never got to it until this year.

So I reached out to Saathvik. He's 14 and, lucky for us, is still curious about this topic.

SAATHVIK: I still want an answer.

MOLLY BLOOM: So, Saathvik, what makes you curious about the human body and the way we regulate our body temperature now?

SAATHVIK: Because my parents are both doctors, I've always been curious about biology. And when I came up with this question and asked them, they thought that we should have reached out to you.

MOLLY BLOOM: That is very cool. Thank you for doing that. Sorry it took so long. So if you could choose between feeling very hot, like being somewhere where it's super hot outside or being somewhere where it's super cold, what would you choose?

SAATHVIK: I feel like I'm pretty adapted to both extremes, because where my school is, we're at the base of Mount Diablo, so in the morning. It's always freezing. But then I still live in California, so in those summers, it's always hot. But I think if I really had to choose, I'd go with always being hot.

MOLLY BLOOM: Why is that, do you think?

SAATHVIK: I don't know. I feel like I have a lot more experience being hot than being cold.

MOLLY BLOOM: And what are your favorite ways to cope when it's hot out?

SAATHVIK: For the most part, I do indulge in ice cream and popsicles sometimes, but what I really like to do is just carry around a massive water bottle filled with ice water and refill it constantly.

MOLLY BLOOM: And I know that you're still really interested in science. What kind of science are you most excited about?

SAATHVIK: Well, before I was really interested in biology and zoology, but because six whole years have passed, my scientific interests have changed a lot. And right now I'm more interested in physics and math.

MOLLY BLOOM: What got you interested in physics and math?

SAATHVIK: I don't know. I think it was something about math being the building block of all of science, and physics being the closest thing to math.

MOLLY BLOOM: Well, before we get to the heart of your original question, let's figure out why our bodies are so hot in the first place.


SAATHVIK: 98.6 degrees is hot. If it were that hot outside, you'd be sweaty and want to go jump in a nice, cold pool.

MOLLY BLOOM: But the insides of our bodies are that hot all the time, and that's because there's a lot going on in there that creates heat.

SAATHVIK: Your heart is pumping blood through your body.

MOLLY BLOOM: Your lungs are moving air in and out.


SAATHVIK: Your stomach is digesting food.


MOLLY BLOOM: Cells, your body's tiny building blocks, are turning that food into energy, and that helps them do what they need to do, like grow hair.


SAATHVIK: Heal cuts.


MOLLY BLOOM: Power your brain.


SAATHVIK: And there's so much more going on in your organs and your bloodstream.

MOLLY BLOOM: Your body is a hopping and very hot place. That's because all of these activities create heat-- a lot of heat.

SAATHVIK: So much heat that your body has to spend a lot of time getting rid of it in order to stay around 98 degrees.

MOLLY BLOOM: When your body is nice and balanced and running as it should, that's called homeostasis.

SAATHVIK: It's a fancy word that just means everything in the body is working as it should. All's good-- homeostasis. There's a part of your brain that helps with homeostasis by keeping track of your body temperature. It also makes sure your body is doing what it needs to do to stay cool.

MOLLY BLOOM: It's called the hypothalamus, and it's always working hard to help you do things like pump blood and sweat. I actually have a workout video for us to try, hosted by the hypothalamus herself. Let's sweat it out.


HYPOTHALAMISSY: Hi, I'm Hypothalamissy, the hypothalamus, and welcome to the Homeostasisize Studio. I hope that body of yours is ready to move, because we are ready to beat the heat. Ooh, do you feel that? It's getting hot in here, so we better bust out our moves.

Just follow my simple, step-by-step instructions, and you'll be sweating it out in no time. Let's start by getting that blood pumping to the skin. And pump. And pump. And pump. And pump.

And as the blood flows from the surface of your skin, it's able to dump some of that heat from inside the body. Pump and dump. Pump and dump.

Who cares if your face is a little red? That just means you're a master of the pump and dump. Ooh, I can feel the heat radiating off your skin from here.

Now it's time to get wet with sweat. Squeeze that sweat out. Secrete. Secrete. Secrete. And as that sweat dries and evaporates, it takes the heat with it. Bye-bye, heat. Secrete. Bye-bye, heat. Secrete.

Ooh, very good. You're getting that heat out of your body like a pro. Time for our last move. It's called the booty shake. You bend your knees just a little bit and shake that rump from side to side. Uh-huh. Ooh-hoo Ah-hah. Ah-ah. Get down with the music. Ooh-ooh. Yeah.

You're probably wondering, how does this get rid of heat from my body? The answer, it doesn't. But it's a great way to elevate your levels of funkiness.

And that's all for today. You've done an amazing workout. You've pumped and dumped. I saw you secrete to beat the heat. And now your body is just the right temperature. I'm the hypothalamus, and you're welcome in my Homeostasisize Studio anytime. Bye-bye, and don't forget to hydrate.

MOLLY BLOOM: Ooh, I am sweaty.

SAATHVIK: Me, too. That was a workout.

MOLLY BLOOM: So our body is able to dump some of the heat it makes by pumping more blood near the surface of our skin. That blood carries heat with it, and the air around our skin soaks up that extra heat, cooling us off.

SAATHVIK: We also lose heat by sweating. When the little droplets of sweat evaporate from our skin, the water and some of the heat float off into the air. That cools us too.

MOLLY BLOOM: Our bodies are able to do this best when the air is 70 degrees cooler than the insides of our bodies.

SAATHVIK: Huh. So as it gets hotter than 70 degrees outside, it becomes harder for our bodies to get rid of extra heat these ways.

MOLLY BLOOM: Yes, especially when it's humid. When air is humid, it means there's already a lot of water floating around in it. So it's harder for the water in our sweat to evaporate, because the air already has plenty of water, and it doesn't need more.

SAATHVIK: So the usual techniques of cooling us off don't work as well when it's humid and hot, so we feel uncomfortably hot.


HYPOTHALAMISSY: Hi, I'm the hypothalamus, and welcome to the Homeostasisize Studio. Ooh, it's hot in here-- like, really hot. OK, let's try our tried and true methods. Get that blood to the skin.

We'll pump and dump.


And pump. And dump. Ooh. I'm pumping, but the dumping isn't working so well. Ugh. I'm so hot.

OK. Ooh. Let's try sweating. Secrete. Bye-bye, heat. Secrete. Bye-bye, heat. Ooh.

The sweat isn't evaporating. I'm so hot. Oh. Ugh. You know what? Forget it. I'm going to the pool.

MOLLY BLOOM: So when it's hot and humid outside, it's harder for our bodies to keep that homeostasis, and we feel hotter.

SAATHVIK: The answer to my question-- finally. Bodies are 98.6 degrees, because they produce a lot of heat. But we need to constantly dump heat to feel cool.

MOLLY BLOOM: Yes and if it's 98 degrees outside, we can't dump heat very well. We're going to hear what happens in our bodies when our temperature goes above the normal, but first, something else for you to puzzle over. It's time for the--



WHISPERING VOICE: Mystery sound.

MOLLY BLOOM: Saathvik, are you ready for the mystery sound?


MOLLY BLOOM: Wonderful. Here it is.


Hmm, what do you think?

SAATHVIK: Well, I mean, it definitely sounds like something grinding, so I'd say a table sliding against a concrete floor.

MOLLY BLOOM: Ooh, very good guess. Should we hear it again?


MOLLY BLOOM: OK, here it is.


Any new thoughts?

SAATHVIK: I mean, it does seem to be going back and forth, so maybe two really rough rocks rubbing against each other.

MOLLY BLOOM: Ooh, very nice guess. I have no idea what this is, either. Why do you think someone might be rubbing rocks against each other? [LAUGHS]

SAATHVIK: I have no idea. Maybe to grind them to a powder, like a mortar and pestle.

MOLLY BLOOM: Ooh. Well, we're going to hear it, get another chance to guess, and hear the answer after the credits. So stick around.


Listeners, we love getting mail from you. Head to brainson.org/contact to send us your mystery sounds, drawings, and questions.

SAATHVIK: Like this one.

CHILD 2: Why does ice help with swelling?

MOLLY BLOOM: You can find answers to questions like these on the Moment of Um podcast, a short dose of facts and fun every weekday. Again, that's brainson.org.

NARRATOR: Brains On Universe is a family of podcasts for kids and their adults. And since you're a fan of Brains On, we know you'll love the other shows in our universe. Come on. Let's explore.


ELECTRONIC VOICE: Entering Brains On Universe.


So many podcasts.


Brains On.


Smash Boom Best.


Forever Ago.


[GASPS] Picking up signal. Smash Boom Best, a debate show. What are they arguing about this time? Tomatoes versus potatoes?


MAN 2: I was just remembering in 1949--



MAN 2: --the Mr. Potato Head went into production--


MAN 2: --a pivotal toy in a lot of people's childhood. And I was googling right now Mr. Tomato Head.


MAN 2: And the first thing that comes up is, did you mean Mr. Potato Head?




[GASPS] Lorp. Signal down.



Need Smash Boom Best. Now.


NARRATOR: Search for Smash Boom Best wherever you get your podcasts.

MAN 3: Ba ba, ba ba, ba ba, ba ba ba, ba, Brains On.

SAATHVIK: You're listening to Brains On. I'm Saathvik.

MOLLY BLOOM: And I'm Molly.

SAATHVIK: So far, we've found that it's hot inside our bodies, because they're really busy and make lots of heat naturally.

MOLLY BLOOM: And we actually have to be dumping heat through our skin to stay at homeostasis, that nice 98.6 degrees. But even though we talk about 98.6 as a normal temperature, it's really just an average. SAATHVIK, how would you describe what an average is?

SAATHVIK: Well, to get the average when you have a set of data points, you add up all of the values and divide by the number of points you have.

MOLLY BLOOM: Oh, very good. That math knowledge is coming in very handy today. So that 98.6 number was first put out there by a German scientist named Carl Reinhold August Wunderlich. He published it in 1868, and in his report, he said 37 degrees Celsius--

SAATHVIK: Which is the same as 98.6 degrees Fahrenheit.

MOLLY BLOOM: --was the average temperature. He said he took multiple temperature readings from 25,000 patients.

SAATHVIK: That is a lot of data.

MOLLY BLOOM: Yes. Some were a little hotter than 98.6, and some were a little cooler. But 98.6 was the average.

SAATHVIK: But several groups of scientists have been studying this more recently, and they've found that our average temperature is now closer to 97.9 degrees.

MOLLY BLOOM: And we need to point out that our body temperatures do change over the course of a day.

SAATHVIK: Our bodies are coolest in the early morning and warmest in the late afternoon.

MOLLY BLOOM: There are also differences between male and female bodies, differences based on age and body size.

SAATHVIK: There are a bunch of reasons that our average body temperature is different now than in 1868.

MOLLY BLOOM: One reason for this is that we have fewer infections now, thanks to a more sanitized world, and our bodies are warmer when they're fighting infections.

SAATHVIK: Many of us move our bodies less overall than people used to, and we also have better medicines and better nutrition.

MOLLY BLOOM: So even though we're a teeny bit cooler, we're still warm enough to keep our bodies running smoothly.

CHILDREN: Brains On.

MOLLY BLOOM: But sometimes our bodies really want to warm up, climbing to 100, 101, or even higher. That's what we call a fever. And it turns out they're super important to help us fight off germs.


JOE ALCOCK: I am Joe Alcock. I'm a professor of Emergency Medicine at the University of New Mexico, and I work in the ER there in a big, urban hospital in Albuquerque. And I also do research. If a normal body temperature is thought to be about 98.6 degrees Fahrenheit, a fever would be over 100 degrees Fahrenheit. The question is, why does that happen? And why did that evolve?

Fever has evolved on the planet in a bunch of different kinds of creatures. It's not just us. If your dog or your cat gets sick, they will also have a higher body temperature than what is typical for them. And what's really amazing is that even cold-blooded animals, they don't have a way of actually increasing their body temperature internally, but they can actually increase their body temperature by seeking out warmth. So lizards that get sick with a bacteria, they'll actually go and sit in the sun, and they'll warm their body temperatures higher than what nonsick lizards do.

But even some animals without backbones do this too. So there's a Senegalese grasshopper that when it gets sick, it seeks out warmer places, and the body temperature of the grasshopper goes up. Honeybees, when they get sick in their colony, what they do is they will actually shiver their wings. They don't fly. They just engage their little wing muscles, and they basically shiver. And that increases the temperature of the entire hive.

There's something similar that happens in worms. There's something similar that happens in snails. OK, so the last common ancestor of birds and lizards and people and worms is about four million years ago. So that means that fevers have been around probably on this planet for about four million years, maybe even longer than that.

So for fever to be so widespread in the animal kingdom, then that really is evidence that fevers might be doing something useful for the animals that show a higher body temperature. People are still doing some experiments to figure out exactly why this happens. But it turns out that when you are infected with something, and this could be a virus or when a bacteria is causing an infection, to be a successful infection, the invader has to reproduce. So one bacterium has to become two, has to become four.

But it turns out that when you increase the temperature, it makes it harder for bacteria to replicate. So it makes it harder for one to become two. From your point of view, when you have a bacterial infection, it can be good to have the temperature be a little hotter, because that makes life a little bit tougher for the bacteria. It also makes life a little bit tougher for you, but that's a price that you are willing to pay to get over an infection.

MOLLY BLOOM: So next time you have a fever, and you feel achy and lousy, just know that your body is doing something pretty amazing and pretty ancient to keep you healthy.


Our bodies are always making heat, but there's a part of our brain called the hypothalamus that keeps our body temperature right where it needs to be, around 97 or 98 degrees.

SAATHVIK: Our body can get rid of heat by pumping our blood closer to the surface of our skin and by sweating.

MOLLY BLOOM: Our bodies like to be so warm because the heat keeps germs in check.

SAATHVIK: And when we get sick, we get warmer to fight off the germs. This is a fever.

MOLLY BLOOM: And it turns out lots of animals have evolved to get fevers, too. That's it for this episode of Brains On.

SAATHVIK: This episode was written by--

MOLLY BLOOM: Molly Bloom.

SAATHVIK: And produced by--

ROSIE DUPONT: Rosie duPont.

SAATHVIK: Our editors are--

SANDEN TOTTEN: Sanden Totten--


SHAHIA FARZAN Shahla Farzan.

SAATHVIK: Fact checking by--

KATIE REUTHER: Katie Reuther.

MOLLY BLOOM: We had engineering help from Michael Peterson and Derek Ramirez with sound design by--

RACHEL BREES: Rachel Brees.

MOLLY BLOOM: Original theme music by--

MARC SANCHEZ: Marc Sanchez.

SAATHVIK: We had production help from the rest of the Brains On Universe team.

ANNA GOLDFIELD: Anna Goldfield.

NICO GONZALEZ WISLER: Nico Gonzalez Wisler.

RUBY GUTHRIE: Ruby Guthrie.

LAUREN HUMPERT: Lauren Humpert.

JOSHUA RAY: Joshua Ray.

MARC SANCHEZ: Marc Sacnhez.

CHARLOTTE TRAVER: Charlotte Traver.

ANNA WEGGEL: Anna Weggel.


ARON WOLDESLASSSIE: Aron Woldeslassie.

MOLLY BLOOM: Beth Perlman is our executive producer, and the executives in charge of APM Studios are Chandra Kavati and Joanne Griffith. Special thanks to Sruthi Coturu.

SAATHVIK: Brains On is a nonprofit public radio program.

MOLLY BLOOM: There are lots of ways to support the show. Subscribe to Brains On Universe on YouTube, where you can watch animated versions of some of your favorite episodes or head to brainson.org.

SAATHVIK: While you're there, you can send us mystery sounds, drawings, and questions.

MOLLY BLOOM: OK, SAATHVIK, are you ready to listen to that mystery sound again?

SAATHVIK: Ready as I'll ever be, Molly.

MOLLY BLOOM: [LAUGHS] OK, here it is.


What are your new thoughts?

SAATHVIK: I think I am going to stick with my mortar and pestle answer, but I'm also pretty unsure, because it does sound a lot heavier than a mortar and pestle.

MOLLY BLOOM: Maybe it's a giant mortar and pestle used by, um, Bigfoot?



That's probably not right. I think you're probably closer than I am. Should we hear the answer?


MOLLY BLOOM: OK. Here it is.

MILES: Hi, my name is Miles, and I live in Atlantic Highlands, New Jersey, and that was the sound of me waxing my surfboard.

MOLLY BLOOM: Oh, my goodness. Waxing a surfboard. Have you ever waxed a surfboard before?

SAATHVIK: I have not, and never in a million years would I have thought of that.

MOLLY BLOOM: [LAUGHS] That was so tricky. OK, so when you wax a surfboard, you have, like, this hard piece of wax that you rub on the bottom of it. So it's like hard, almost like a rock. And the surfboard itself is kind of bumpy and hard, and so that's making that noise. Pretty cool, huh?


MOLLY BLOOM: You're in California. Would you ever like to try surfing?

SAATHVIK: I mean, maybe, just so I can wax a surfboard.



Now it's time for the Brains Honor Roll. These are the incredible kids who keep the show going with their questions, ideas, mystery sounds, drawings, and high fives.


Next week, we'll be back with an episode about how trees help cities.

SAATHVIK: Thanks for listening.

Transcription services provided by 3Play Media.