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Here on SciShow, we've talked a lot about poop and the science behind one of the more private aspects of our lives. So, our gift to you is a collection of just SOME of our poop-related episodes. Happy Holidays!

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*scishow theme music*

Pooping! Everybody does it. Which maybe makes it seem like kind of a mundane thing. But we've actually made a lot of poop-related episodes on scishow because there's a ton of interesting science behind it. There's the feces itself. How our bodies process it, the technology we use to handle it, and also some ways we encounter, or seek out, other animals' poop.

So here's collection of just some of our episodes about poop. And if you're currently sitting around with your family for the holidays, maybe just unplug your headphones and turn up the volume so that everyone can experience the glorious science of poop. 

You really wouldn't want them to miss out, would you?

It turns out that the one of the most commonly googled questions in the world is: why is my poop green? So, up first, 2014 Hank is gonna tell us why that happens. 

2014 Hank:
You can ask the Internet anything you want, like "how big is the universe?" or "what's the average lifespan of an Olive Ridley sea turtle?" or "why are sloths so lazy?" But many of you turned to the wisdom of the Internet to explain what you find in the toilet bowl. 

According to our friends at Google, one of the most commonly googled questions in the world, at least in English, is: why is my poop green?

And, hey, fair enough. We're all about fostering curiosity here and what's more fascinating than the human body?

So in order to answer this question, we should start out, really, with: why is poop normally brown? The brown color of most mammalian feces comes from a substance called "bilirubin," which is produced by your liver when it processes dead, used-up red blood cells and then prepares them to be excreted. The bilirubin is actually made from hemoglobin, the protein that your blood cells use to ferry oxygen around your body. 

But even though your red blood cells are red, the bilirubin itself is yellow. And it's absorbed by your liver and excreted as bile, which is yellow-ish green because of all the bilirubin in it. The liver secretes bile into your small intestine where its main job...

 (02:00) to (04:00) to digest fats, breaking down lipid molecules into fatty acids. But since the bile is on its way down your body anyway, it's also chock full of waste material. Including that bilirubin.

So, the fact is, because of all the bile in it, your feces actually start out a yellow-ish green color. Typically, as this junk travels through your digestive system, the bilirubin is broken down by your gut bacteria--those wonderful, microbial minions that live inside your intestines and help you absorb nutrients while decomposing waste.

These bacteria eat the bilirubin and metabolize it into a byproduct that's colorless. But when that byproduct reacts with oxygen, it turns brown, forming a pigment called stercobilin. Stercobilin is what makes your poop brown. 

So, if your poop is green, it means that it went through your digestive system too fast and the bacteria didn't have time to digest your bilirubin into its byproducts. 

So if you notice that the stuff's a lil greener after you take a laxative or have a touch of food poisoning or maybe super hungover--something else might be going on that's hurrying your poop along too quickly. So the occasional green turd is probably nothing to worry about. 

But if it's always that color, it could mean that your feces aren't spending enough time inside of you for your intestines to absorb all the nutrients you need. 

Mystery solved!

Well, that's one question answered. But I have so many other questions about poop. Like, when I go to the local wing joint for Wing Wednesdays and I eat a bunch of super spicy wings, I know that I might be signing up to re-experience that spiciness about twelve hours later. But why does that happen? It doesn't make any sense. 

Well, Olivia has the answer. 

Lots of people love the painful deliciousness of spicy food. At least, when it's in your mouth. But why do foods that burn going in also burn going out?

Well, turns out it's for the same biological reasons. Because in some ways your butt is a lot like your mouth. 

Lots of spicy foods get their pungency from a chemical called capsaicin. It's produced by the plants in the genus Capsicum...

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...which includes chili peppers and their relatives. Capsaicin binds to a receptor expressed all over your nervous system called TRPV1. It has a handful of jobs, but it's best known for its role in pain. When there are risky levels of heat, this receptor tells your danger-sensing cells and other chemical messengers to send signals to your brain which can make you feel pain. And because TRPV1 also interacts with capsaicin molecules, you feel the burn of habaneros in your mouth. 

According to you nervous system, hot peppers actually feel hot... temperature-wise.  

But TRPV1 receptors are in lots of places and that includes your anus. See, the capsaicin you eat doesn't get completely digested. Some of it does get absorbed by your body and sent to your liver to be broken down, but some of it sticks around and gets pooped out. Which means the TRPV1 receptors in your anus can interact with capsaicin just like the ones in your mouth. Ouch.

There are actually TRPV1 receptors all along your digestive tract, which is why some people get cramps or discomfort from spicy foods.  

And when your body's being told that things are way too hot, it usually tries to cool down. You sweat, your nose runs, and your mouth waters in an attempt to cool off and flush the offending substance. Your intestines might get in on the action and throw in some water, too. And that's why your spicy-food poop is sometimes... looser than normal. 

Now, consuming capsaicin regularly can actually make your body desensitized, possibly by decreasing the amount of certain chemical messengers so your brain reports less pain. Like, oh! Okay, this happens all the time. There's no fire or real danger. Everything's fine!

In fact, using low does of capsaicin for pain relief is an active area of research. 

So go ahead and order those jalapeños on your pizza, because maybe eating spicy foods more regularly will help with those burning bowel movements. Even though it might not be super pleasant in the meantime. 

Hm. Finding out that my mouth is in any way like my butt makes me regret asking. Just a little. 

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So now I'm a little afraid to ask, but why does poop sometimes float? 

Have you ever looked down after doing your business and noticed that your poop is floating? Maybe this comes and goes, or maybe your poop is always buoyant. You might have even worried that this was a problem.

Like, if you look it up online, a lot of reputable websites will mention that one reason your poop floats is because of fat. Fat is less dense than water, and so the thinking goes: if you have enough of it in your poop, that's probably why it floats. 

But fatty stools--what doctors like to call steatorrheic stools--happen because you aren't absorbing the fat that you eat. And that's usually a sign of a really serious problem. Like pancreatic cancer or cystic fibrosis.

Back in the early '70s, Micheal Levitt and William Duane, a pair of researchers at the University of Minnesota, were annoyed by this assumption that fat is why feces float. They suspected that trapped gas was the mroe likely culprit. After all, about fifteen percent of perfectly healthy people, including Duane, consistently had bobbers. 

So they decided to investigate and kindly wrote up their results in the prestigous New England Journal of Medicine in a paper called "Floating Stools--Flatus versus Fat." Thirty-three healthy volunteers, some with floaters, donated samples, along with six patients with steatorrhea.

The scientists put the specimens in flasks with water to see if they sank. If not, they added pressure to squeeze out any gas and checked to see if anything still floated. But even the fattiest poops sank to the bottom when degassed. In fact, the scientists determined that it's almost impossible for a bowel movement to float just because of its fat content. You would need it to be half fat.

So, gas creates most of the uplift. But what is it? And where is it coming from?

Luckily, our gutsy duo also analyzed the poo samples with gas chromatography to separate out the compounds and identify the mysterious gas. It turns out that a big part is methane...

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...the same stuff in natural gas. It's made by special bacteria in the colon called methanogens. Everybody has some of these bugs. But some people seem to have way more. Like William Duane, who admitted in his paper that he produced methane at near record proportions. 

Methanogens run on hydrogen and can ferment fiber in the gut, a bit like a cow can. That means you get more energy out of fiber-rich food, like beans. So don't believe everything you read online about your poop. 

If you have floaters, you could just be gassy. And if you have fatty poop that happens to float, well, that's because of gas, too. 

Aaahhh, I guess it does make sense that gas would be involved in one of these answers. In the history of humanity, we've certainly come a long way since we were just pooping out in the woods somewhere. And it took us a long time to arrive at the modern sewage treatment systems we have today.

It also took an event that is now known as "The Big Stink." So here's a longer episode that explains where the poop goes after you flush and how we treat it to keep ourselves safe. 

The year is 1858, the city is London, an oppressive heatwave is scorching the city's 2.5 million residents, and the Thames River, where for centuries Londoners have disposed of their waste, begins to stink. Bad. Like really bad. 

Water levels drop as raw sewage bakes in the sun, casting a fetid smell so powerful that it's said to strike men down! It would go down in history as The Big Stink, all three of those words capitalized, and as unpleasant as it was, it's remembered today as a tipping point of sorts that eventually ushered in the age of modern sewage treatment.

Humans have always peed and pooped, of course, but where it goes after we've done our business has changed a lot. Especially in the last 160 years. 

As a result of The Great Stink, London ended up building more than 20,000 kilometers of underground sewers to channel human waste downstream where it could be released, but still untreated. And it turns out that just diluting and dispersing sewage and hoping for the best isn't great public health policy. I mean, the Greeks and Romans were...

 (10:00) to (12:00)

...doing the same thing 2,000 years earlier. Not exactly progress.

Over time, it became clear that human waste not only had to be disposed of, but it also had to be rid of the polutants that are harmful to humans and the environment. 

But what is it that makes poop and pee so dangerous in the first place? I mean, the stuff comes from your body, so it doesn't make you sick while it's in there. Why is there a problem?

Well, we're learning a lot these days about the human microbiome, the wonderful habitat that your body provides for literally trillions of microorganisms. And it includes your waste-making parts. Like, there's growing evidence that your urine isn't actually sterile, though for a long time we thought it was. And your intestines are populated with huge bacterial colonies that you actually need in there to help digest your food and fend off some infections. 

Unfortunately, we're not all healthy one hundred percent of the time, and the viruses and bacteria that make us sick are found in our poop. So human waste can contain all kinds of pathogens, like salmonella and E. col, and the microorganisms that cause cholera, typhoid, and hepatitis. Then you got parasites like hookworms and protozoa and giardia, which campers may be familiar with after drinking improperly filtered water. 

And it's more than just a problem for humans. Wastewater also contains lots of nitrogen and phosphates that can harm wildlife and water quality if released into the environment. 

So while a lot of us take it for granted, dealing with these challenges is harder than you might think. Lack of access to clean water and flushing toilets is a serious public health problem in many developing countries. The treatment processes that so many of us use aren't available to well over a billion people.

The wastewater journey usually takes one of two directions. It either heads by way of a pipe to your town's local sewer, or into a septic tank right there by your own home.

Even though many of us live in big cities, backyard septic systems are actually very common. As many as twenty-five percent of people in North America depend on them to process their waste and distribute it into the ground. 

Scientists learned long ago that the anaerobic digestion of sewage by microorganisms can be extremely effective in breaking down human waste and eliminating pathogens. The word "septic" actually comes from the Latin for the...

 (12:00) to (14:00)

...bacterial action that you know as rotting. All the stuff that goes down your toilet--which wastewater management types call black water--and all the stuff that goes down the drain in your sink and your shower--or gray water--flows into that tank, which usually holds about four thousand liters of liquid. Then, in the span of about two days, naturally occurring anaerobic bacteria get to work producing solids in the wastewater into liquids and a kinf of greasy, fatty residue. 

The insoluble particles will sink to the bottom and form the sludge, while the fats form a layer at the top of the tank. And, yes, I realize that I am probably ruining your day at this point. 

But within 48 hours the bacteria will break down the wasterwater in the septic tank by about forty percent, and they stay in the water when it flows out of the tank into an underground drainage field. Here, the bacteria continue to do their thing while the soil acts as a biological filter of sorts, keeping any harmful organic material from reaching the groundwater supply. 

Soil contains oxygen, which allows aerobic bacteria to enter the mix, and it also contains some of the phosphorous and forms of nitrogen like ammonia, which plantlife really enjoys. 

So, yeah, if you're off the sewer grid, all that's going on in your backyard right at this very moment. Kinda makes you not wanna walk around in your bare feet anymore, doesn't it.

But for those of you connected to a sewer system: what kind of magical journey awaits your waste? And, because I know some of you are curious, what happens to the stuff that you flush down your toilet that isn't your own excreta?

Well, before it flows into your local treatment plant, wastewater goes through a screen of metal rods that catches all the big stuff--sticks and clothing and small animals and other weird non-poo that finds its way into the sewers--so that none of that gets into the moving parts of the facility.

From there, it's on to the settling tank, a.k.a. the "grit chamber," which is where solids like sand and gravel that have been picked up along the way settle to the bottom. It's also at one of these two early treatment stations where other flushables are removed. And there are literally tons of this stuff. According to one estimate, as much as fifty percent of the so-called "non-dispersable material" in wastewater is paper towels from public restrooms. Another 25 percent is...

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...nothin' but baby wipes. And the remaining 25 percent is a mixture of condoms and cosmetic wipes and tampon applicators and random things that toddlers probably got their hands on. 

The process for removing them varies depending on the system, but usually involves another mechanical screen or skimmer.

The real treatment begins at the next stage, called the "primary clarifier." This circular basin slows down the sewage so that the heavier organic solids fall to the bottom. The solids at the bottom will be removed and transferred to a digester, sludge press, or drying bed, which we'll get to, I promise. 

But back to the primary clarifier: Most facilities also use a skimmer here to remove fats and oils and grease that float to the top. These substances, most of which come from cooking materials or road runoff, will also be diverted into the digester. The largest change in the science of wastewater treatment over the last few decades is that the process used to stop right here. People were like, "Hey, okay, we got the sludge out. The water's good to go back in the rivers."

Except it's not. 

As we discussed earlier, the wastewater is still full of potentially dangerous pathogens which is why we now have what's called secondary treatment. Wastewater from the clarifier flows into an aeration basin, where air is added to foster an efficient environment for microorganisms that, unlike those used in most septic systems, consume both organic matter and oxygen. 

At this point, the water is known as "activated sludge," and it looks like dark mud. It's rich with active bacteria and protozoans that go to town on the oxygen-rich water. 

From there, the water heads to a secondary clarifier and yet another sedimentation basin--its one last time for any leftover heavy stuff to fall to the bottom, where it's pumped to the digester. 

And while the name is kinda gross in this context, the digester does pretty much what it sounds like. It eats your poop. Inside, it has anaerobic environment similar to a septic tank, where bacteria can break down sludge, reducing the volume, the odors, and most importantly, disease-causing organisms. 

Some facilities use drying beds for this step, but either way, the material left over from this process is called biosolids...

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...or "treated sewage sludge." It wasn't so long ago that we took these biosolids and tossed them into a landfill once they were through treatment. 

But today, this leftover, dry sludge is often used as fertilizer for agriculture or used to make compost mixtures that are sold commercially. So, again, yes, you may have used the very diluted byproduct of human poop to grow the veggies in your garden. We here at scishow love recycling. 

After all those solids are taken care of, we still have a bunch of wastewater left over. Now, that needs be disinfected before being released into the wild, also known as our drinking supply. 

There are lots of ways to do this. Some facilities use chlorine to kill any harmful bacteria still left in the water. Chlorine can be introduced as a gas or a salt. Either way, it reacts with the water to form hypochlorous acid, which ultimately breaks down bacteria's cell membranes and kills them.

And chlorine is really effective, but if you live in a town that uses it, you know that it does not taste good at all. Chlorine also can be dangerous for organisms in the rivers and streams that receive the wastewater, which why some places now dechlorinate water after chlorinating it. This is done using sulfur dioxide gas, which quickly turns chlorine compounds into chloride ions, which makes the water less toxic. 

A more expensive but less chlorine-y method involves using ultraviolet light, which effectively sterilizes microorganisms, changing their genetic composition enough that they can no longer reproduce.

Some newer wastewater treatment plants now even go an extra step with what's called tertiary treatment. It can be a physical process, where water is run through sand filters to reduce any remaining solids. Or it can be done biologically. Some places divert the water to natural wetlands where plants remove excess nutrients.

But this is the end of the line. No matter what the disinfectant process, the effluent will be released, usually into a nearby river or creek, ready for you to possibly drink again someday. Your pee and poop has come full circle!

Aaahhh, so refreshing. So it turns out that the history of poop is rich with stories.

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Apparently, in the mid-nineteenth century, there was a worldwide craze for the poop of a particular species of bird. 

Throughout history, humans have prized a lot of animals for making products that we find beautiful or useful. Silkworms for their silk, whales for their blubber, sheep for their wool. 

But in the mid-1800s, a bird called the guanay cormorant was considered the most valuable in the world. It was nicknamed the "billion dollar bird" because its poop, or guano, was an amazing fertilizer.

At the beginning of the nineteenth century, populations were exploding, and all of those people needed to eat. Large-scale agriculture had been booming for a few hundred years, and soils were quickly getting depleted of nutrients. 

Enter Alexander von Humboldt, European explorer extraordinaire. While exploring the coast of Peru in 1802, von Humboldt came across workers unloading a shipment of guano from the Chincha Islands. Indigenous cultures in the area had been using the guano on these islands as a source of fertilizer for hundreds of years, so von Humboldt definitely did not discover guano. But, like any good explorer, he took a sample of it back with him to Europe. 

And at the time, people didn't know much about the science of fertilizers. Farmers recognized that adding things like ground-up bones, ash, or feces to soil helped plants grow, but scientists weren't sure why this worked. We now know the answer is elementary.

Actually elementary. Like, literally it's elements that are involved. 

These materials have a lot of nitrogen, phosphorous, and other elements that help plants grow. Plants use nitrogen to make important proteins and pigments like chlorphyll, which let them absorb sunlight and do photosynthesis and not die. Phosphorous is needed to make DNA and RNA, as well as cell membranes and the molecules that all living things use to make and store energy. Without these elements, plants can't make more cells or even function with the cells they have. 

But even if scientists had known this at the time, it wouldn't have helped them very much. There just weren't many good sources of nitrogen and phosphorous that plants could use. There's plenty of nitrogen in the air, of course, but plants can't just suck it in like carbon dioxide. 

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They usually rely on bacteria in the soil to convert or "fix" the nitrogen into a different form, like nitrate or ammonium, that they can actually use. If the soil has been exhausted of these chemicals, it's really hard to grow crops. Which brings us back to the Chincha Islands. 

Turns out, bird guano is amazingly rich in these sources of fixed nitrogen and phosphorous. In fact, it's even better than cow dung or horse dung. Cows and horses only eat plants, but guanay cormorants feast almost entirely on anchovy-like fish. That protein-rich diet means lots of nitrogen eventually shows up in their poop. Plus, bird poop is really poop and pee combined, so that's extra nitrogen and phosphorous that other animal dung leaves out. 

As for why this natural fertilizer could just pile up, thank the dry climate of coastal Peru. In most of the world, bird poop will simply get washed away. But ocean currents from Antarctica ensure it almost never rains on the islands, allowing guano to harden, locking in layer upon layer upon layer of nitrate-phosphate goodness. 

So when word finally reached Europe that von Humboldt had potentially found a great new source of fertilizer, they went crazy for it. Almost immediately, the Peruvian government started huge mining operations, selling the guano to any country that wanted it.

Other countries got involved, too. In 1856, the United States even went so far as to pass the Guano Islands Act, which allowed American citizens to claim any island they wished, as long as it had guano on it and wasn't already claimed by another country.

Guano miners came from all over the world, harvesting the seemingly limitless resource. But while the guano boom was great for agriculture, it was bad news for the guanay cormorant and other sea birds that called the islands home. Miners ate the birds and their eggs, which was not really smart when you think about, and destroyed their habitat to the point that their populations began to fall. 

By the 1870s, most of the guano on these islands had already been mined. A few decades later, Peru started protecting the remaining cormorants. It's one of the first examples of a government stepping in to protect a natural resource. 

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But for the guano industry, it was too late. With fewer droppings to mine and the discovery of artificial fertilizer, the guano boom went bust. 

German chemist Fritz Haber had figured out how to fix nitrogen from the air and turn it into ammonia, which could in turn be used to develop synthetic fertilizers. These worked just as well, if not better, than guano. And now we've been using these fertilizers for decades to feed billions of people. 

But guano might not have outlived its usefulness just yet. Recently, the rise of organic farming has increased demand for high quality organic fertilizer, and guano is one of the best. 

Now, after all this talk about poop, you might feel the need to go out and get some fresh, non-smelly air. Like maybe by going to the beach and doing some sunbathing. At least if you're in the southern hemisphere, cuz it's winter up here. 

But I am sorry to tell you, you can't escape poop. Up next, Michael's gonna tell you all about how those lovely white sandy beaches are nothing but a bunch of ground-up fish poop. 

Let's say you wanted to make yourself a white, sandy beach. You know, like the ones in Hawaii or the islands in the South Pacific where people take luxurious vacations and lots of Instagram photos. 

It turns out that the recipe for that picturesque, white sand is pretty simple. There are a couple steps involved, but just one main ingredient: poop. 

The first thing you'll need is a coral reef. It's location specific and unfortunately there's no store-bought alternative. Coral reefs are made up of thousands of tiny polyps, which are squishy sacks with tentacles that can sting and catch prey. Kind of like sea anemones. These polyps are anchored to a sturdy skeleton they make by laying down a crystallized form of calcium carbonate called aragonite, which is a bright white color. 

Reefs are found in parts of the ocean that are considered to be oligotrophic, which means there aren't that many tasty nutrients floating around, like phosphate or nitrate. And coral polyps aren't exactly mobile, so if they were left to their own devices, they might starve. But coral polyps aren't alone. They get help from an algae called zooxanthellae that lives symbiotically in their tissues. These algae can photosynthesize, chanding sunlight, carbon dioxide, and water into food energy, and share a large chunk...

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...of that energy with the coral. As coral polyps get more energy, they build out their skeletons and grow the reef. And these algae are also what give the coral its bright colors. When you have a thriving coral reef, other undersea plants and animals move in and begin to form a whole ecosystem. 

And to get your white, sandy beach, you need some parrotfish, or more specifically, their poop. Parrotfish get their name for their bright colors and their weird, birdlike beaks. These beaks are actually made of fused teeth called dental plates, which help them graze for their main food source: algae. Parrotfish spend their days scraping away at coral reefs, eating polyps for the zooxanthellae inside and any other algae they can find. They're not super careful or picky, so they end up swallowing mouthfuls of calcium carbonate, too, which they can't digest for nutrients or energy. 

But the parrotfish have a secret weapon. Their throats are hiding another set of teeth called pharyngeal jaws that can grind up the coral skeletons. Like the xenomorph from Alien, but in real life. 

This makes it easier for the chunks to travel through their digestive system. And they get rid of this calcium carbonate in streams of the purest, whitest tropical sand. In fact, according to some studies, one parrotfish can poop out around three hundred kilograms of sand, or more, in a single year.

After some ocean currents stir everything around, you have yourself a pristine, white-sand beach. Perfect for strolling, sunbating, and building sand castles... as long as you don't mind fish poop. 

Well, hopefully we haven't ruined everything for you. Thanks for joining us here on scishow to review our catalog of videos about poop. Honestly, we couldn't even include all of them in one video, so we'll probably have to do a poop compilation number two at some point. 

So if you don't want to miss that, or any of our other videos, go ahead and hit that subscribe button. And I hope you enjoy the rest of your holiday season now that you're armed with all these poop facts that you can go share with your family and friends.

*scishow credits music*