From your first bite to the final flush—everything you ever wanted to know about digestion, poop, and farts
This article is based on But Why: A Podcast for Curious Kids, produced at Vermont Public Radio and hosted by Jane Lindholm. The show tackles the big, silly, and sometimes gross questions that kids are genuinely curious about — and this episode is no exception. Drawing on the podcast’s episode about digestion, as well as a special segment from Chicago Public Radio’s Curious City, this article breaks down how the human digestive system works in a way that is fun, accessible, and easy to understand for younger readers. From how your body turns food into poop, to what happens after you flush the toilet, to why farts smell the way they do—consider this your kid-friendly guide to one of the body’s most essential (and fascinating) functions.
Your body’s amazing food-processing machine
Everything starts the moment you take a bite. Your saliva immediately gets to work breaking food down, and once you swallow, your stomach takes over like a blender—churning everything together with powerful digestive juices to make it even smaller. That sloshy mixture then moves into your intestines, where your body soaks up all the useful stuff: nutrients, energy, hydration—everything you need to run, grow, think, and play.
Here’s a mind-blowing fact: if you could uncoil and stretch out the small and large intestines packed inside your body, they’d measure over 25 feet—longer than a city bus.
As explained here, whatever your body can’t absorb has to go somewhere, and that’s where poop comes in. Officially called feces (you might also hear stool or excrement), poop is mostly water, mixed with fiber and dead cells your body has shed along the way. Its texture and shape can vary depending on what you’ve been eating and how healthy you are. The whole process — from bite to flush — typically takes about a day, and it’s honestly remarkable: your body extracts everything valuable from your food and neatly pushes the rest out to make room for more.
Pee works differently. Urine is how your body flushes waste products from your bloodstream, not your digestive tract. The more you drink, the more you pee — and that’s a good thing. Light yellow means you’re well hydrated; dark yellow means it’s time to drink more water.
Just how many germs are in poop?
Here’s a number that’ll blow your mind: about one trillion germs can live in a single gram of poop — roughly the weight of a paperclip. Since there are 28 grams in an ounce, that’s potentially 28 trillion germs in one ounce of poop. For comparison, an ounce weighs about the same as a slice of bread, 11 pennies, or ten ruby-throated hummingbirds!
Not all of those germs will make you sick, but some could — which is exactly why washing your hands with soap after using the bathroom is so important.
Where does your poop go after you flush?
Most cities and densely populated towns have sewage systems with pipes that carry waste away from your home to a water treatment facility. Chicago’s Stickney plant, for example, is the largest sewage treatment plant in the world, covering 400 acres — the equivalent of 302 football fields — and capable of processing a million gallons of sewage per minute.
At a plant like this, the journey goes through several stages. First, large screens filter out big objects that have washed into the sewers — branches, chunks of concrete, and occasionally stranger things like bowling balls, prosthetic legs, turtles, and even loose cash. Then the water sits in large settling tanks, where solids sink to the bottom, and fats float to the top, both of which get skimmed off.
The remaining water enters large tanks full of helpful microorganisms — tiny living things that eat the harmful bacteria and viruses in the sewage, cleaning the water naturally. Scientists carefully monitor these microbes to keep them in balance. Once the water is clean enough, it’s released back into the river system.
The leftover solids go through their own process: they’re dried out, treated with more microorganisms, and eventually turned into a rich compost called biosolids, which can be used to fertilize parks, golf courses, and even home gardens.
If you live in a rural area, the system works differently. Many countryside homes use a septic tank buried in the yard, where solid waste is stored while liquid filters out through an underground field. Every few years, a truck comes to pump out the solids and take them to a treatment facility.
Why do we fart—and why do they smell?
When you eat, you don’t just swallow food—you swallow air too. That air, made up of gases like nitrogen and oxygen, travels through your digestive system. Some of it comes back up as a burp. The rest keeps moving and eventually comes out the other end as a fart (officially called flatus, or flatulence).
On top of the swallowed air, the bacteria in your intestines that help break down your food also produce gases like methane, carbon dioxide, and hydrogen. These build up and can become uncomfortable — that pressure is your body’s signal that it needs to release the gas.
Everyone farts, by the way, about 20 times a day on average. And your body is actually smart enough to tell the difference between gas and solid waste, usually allowing you to release one without the other. Pretty impressive engineering.
As for the smell, that comes down to the bacteria in your lower intestines. When those microbes break down your food, they sometimes produce sulfur-containing gases, which are the especially pungent ones. Diet plays a big role: foods like beans, broccoli, cauliflower, dairy (for some people), and high-fiber foods like wheat and corn tend to produce smellier gas. That said, almost any food can cause gas in the right circumstances. If your stomach is frequently painful or gassy, though, that’s worth mentioning to a doctor.
So there you have it—from the first bite to the final flush, your body runs an incredibly efficient, fascinating system. And now you know exactly what’s happening every step of the way.