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You know what's more like the life than a box of chocolates? Farts-- you really never know what you're going to get. A skunk, rotten eggs, garbage dumpster-- the possibilities are, unfortunately, endless. But why do we fart, and how does your body make so many different smells? The answer is fermentation. This is a giant bioreactor. It's also called a fermentor, and in here, a fungus known as brewer's yeast is transforming water and dead plants into beer. So you know that thing where you eat food and breathe in oxygen to get your energy? That's called respiration. Yeast also need to get energy from food, but there's no oxygen in those tanks. So they have to use a different process called fermentation. Now both yourselves and yeast cells break food down to get energy. But your main by-products are carbon dioxide and water, and yeast's main by-products are carbon dioxide and alcohol. That's how this stuff gets to be beer. And here's where things get weird. Turns out, we all carry a fermentor with us right here in our gut. Except instead of yeast, my gut uses bacteria, and a lot of them. There are 10 times as many bacteria in my intestine as there are human cells in my entire body. And instead of just one species, my gut uses hundreds. And all those bacteria care about is staying alive. Our intestine actually helps them do that, because the temperature is warm and constant, they're sheltered from the environment, and best of all, we provide the food. By the time food reaches our intestine, we've pretty much digested it as best we can. But there's some stuff in here like cellulose, pectin, some starches and complex sugars that our bodies cannot digest. But our gut bacteria can. Just like yeast ferments sugar, our gut bacteria ferments all the stuff that our body can't digest. But it also ferments us. Our gut is coated with mucus, and we're constantly shedding dead gut cells. And both the mucus and the dead gut cells contain protein and carbohydrates that our bacteria can ferment just as well as partially digested beans. Now if we had brewer's yeast in our gut, all that fermentation would produce alcohol, and we'd get drunk anytime we ate anything. Luckily, the main product of our bacterial gut fermentation is not alcohol, but these guys. These are short chain fatty acids. And they're completely digestible by us, which means that our bacteria have taken something that we cannot digest and turned it into something that we can. And we do. We make it into energy. And that is energy we would otherwise have just pooped out. That is why we keep an army of bacteria in our guts. But all that fermentation creates gas-- lots of gas, and not just odorless carbon dioxide-- smelly stuff like hydrogen sulfide, dimethyl sulphide, methanethiol and many others. Now you could mix some of that stuff together to create artificial fart product. But the problem with this is that it doesn't begin to capture the magical complexity of real farts. For one thing, what comes out depends on what you put in. Foods with more indigestible chemicals produce more farts. And everybody's gut bacterial profile is a little different. With 100 trillion cells all metabolizing different things in different ways, how could it not be? We don't really know yet how my gut bacteria differ from yours, or what those differences mean. But we are just starting to figure out that gut bacteria do a little bit more than just help you digest things and make smells. If you take filtered poop extract filled with gut bacteria and whatever else they've been swimming around in from one person and put it into another, that's called a fecal transplant. Fecal transplants sound gross. But as an investigational therapy, they've been pretty effective at getting rid of really, really bad clostridium difficile infections-- ones that have not responded to antibiotics. C. diff. is a bacterium that kills 14,000 Americans a year. A few MIT researchers have partnered with doctors to create a nonprofit poop bank called Open Biome. And what Open Biome does is it sends physicians poop extract filled with glorious gut bacteria from screened, healthy donors for fecal transplants. Ready? We're at the very beginning of understanding the incredible, smellable science in your gut. And until we do, happy farting. [MUSIC PLAYING]