Why do we poop and pee? Pee is waste and extra water that your body does not need, while poop is the leftover parts of food that your body cannot use. Food goes in at your mouth and poop comes out at your anus, at the other end of one long tube. Digestion begins in your mouth, where spit makes food easy to swallow, and once you begin swallowing, the process becomes automatic. After that, food moves along the gut by peristalsis, a wave of muscle squeezing: the muscle behind the food contracts and pushes it forward while the muscle in front relaxes to let it move. Your body makes pee and poop on its own, but you get to choose when to go to the bathroom.

The stomach makes acid that helps break food down, and protein digestion begins here, where acid and an enzyme called pepsin break it down. The stomach churns food with its juices into a thick mix called chyme. It is also a holding chamber: you can eat much faster than the small intestine can digest and absorb, so the stomach passes food on only a little at a time, and after a meal it is usually empty within 2 to 4 hours. Stomach enzymes that digest protein could digest the stomach itself, so a thick coat of bicarbonate-rich mucus forms a barrier that neutralizes acid. Next in line is the liver, which makes bile, a digestive juice that helps break down fats, at roughly half a liter to a liter a day.

The gallbladder stores the bile between meals and squeezes it through the bile ducts into the small intestine when you eat, so the fat-digesting juice arrives just as the food does. Bile acts like dish soap on grease: its salts break fats into tiny droplets (emulsify them) so the body can digest and absorb them. The small intestine then takes nutrients from digested food into the blood, and it takes in water as well. Digestion is chopping nutrients into pieces small enough to cross the gut wall: proteins break into amino acids, fats into fatty acids and glycerol, and carbohydrates into simple sugars. Tiny bumps called villi, about 20 to 40 in every square millimeter, give the small intestine a huge surface for taking in nutrients. The liver also sends a yellow substance called bilirubin out in bile, and bilirubin helps give poop its brown color.

The large intestine takes water out of leftover food and turns it into solid poop. Poop is more than leftover food, because the waste that reaches the large intestine also includes fluid and older cells from the lining of the gut. Bacteria that live in the gut help digest food, and digestion in the large intestine is done by its bacteria, which ferment leftover carbohydrates and make vitamins such as vitamin K that the body absorbs. Gut bacteria are called gut flora, or the microbiome, and trillions live in the large intestine. A single gram of poop, about the weight of a paper clip, can hold from about 100 billion to a trillion germs. The rectum, the last part of the large intestine, holds poop until it is time to go, and two ring-shaped muscles called anal sphincters keep the anus closed. You can choose when to open the outer one.

You have two kidneys, one on each side of your back, just under your ribs, and each kidney is about the size of a fist. Your kidneys clean your blood, taking out waste and extra water to make pee. Blood goes into each kidney through the renal artery and comes out cleaned through the renal vein, and two thin tubes called ureters carry pee from the kidneys to the bladder. Urine is a mix of water and wastes, and it holds high concentrations of urea. In people, a smaller share of urea also leaves in sweat, but the urinary system does most of the work.

Each kidney holds about a million tiny filters called nephrons. In each one, a knot of blood vessels called the glomerulus lets water and small wastes slip through its thin walls while proteins and blood cells stay in the blood. A tube called the tubule then pulls most of the water and the minerals you need back into the blood. Nephrons make pee in three jobs: filtering the blood, taking useful things back into the blood (reabsorption), and adding extra wastes (secretion). Your kidneys filter about 150 quarts of blood in a single day, but only 1 to 2 quarts leave the body as urine. That means well over 98 percent of the fluid that is filtered is put back into the blood, so the kidneys work mostly by sorting what to keep.

The bladder is a hollow, muscular, balloon-shaped organ that acts like a reservoir. An adult bladder can hold about 1.5 to 2 cups of pee, and stretch receptors in the bladder wall send nerve signals to the brain as the bladder fills. People first sense the urge to pee when the bladder holds about 150 milliliters, but the urge is easy to ignore for a while. Two sets of sphincter muscles squeeze the exit tube shut, and the inner ones stay closed until the brain sends a signal to urinate. Then the brain tells the sphincters to relax and tells the muscular bladder wall to tighten, which squeezes urine out through the urethra. You do not control your kidneys, but you do control when you empty your bladder.

Wombats are the only animals known to make poop shaped like cubes, as of November 2026. One idea is that the cube shape stacks well and does not roll, so wombats can pile them up to mark territory and attract mates. Researchers studied intestines from wombats that had died to learn how the cubes form. A wombat's intestine is stiff in some parts and soft in others, and this uneven stretchiness helps mold the poop into cubes. To test the idea, scientists built mimic intestines: simply shoving beads through did not make cubes, but contractions of stiff and soft parts produced flat sides and corners. The wombat poop study won the 2019 Ig Nobel Prize for Physics, an award for research that makes people laugh and then think.

Dung beetles turn poop into a nursery. Many feed on the microbe-rich liquid part of mammals' dung and lay their eggs in balls made mostly of the leftover fibrous material, so the waste of one animal becomes food and a home for another. Bird and bat droppings called guano became a global fertilizer because they are rich in nitrogen, phosphate and potassium, the nutrients plants need. Whale poop floats, so researchers who spot it can scoop it up with a net. Scientists analyzed five whale poop samples and found iron in every one, plus copper (as of February 2025). Researchers do not yet know where the whale poop ligands that make iron usable come from, but suspect bacteria in the whales' stomachs (as of February 2025).

In 1849 the London doctor John Snow published a paper arguing that cholera spread through contaminated water, not through bad air, which went against what most doctors of his day believed. Snow did not win the argument quickly. He failed to convince many in the medical establishment, including William Farr of the General Register Office, who accepted Snow's data but not his theory. Farr changed his mind only after the last London cholera epidemic of 1866. After the 1854 outbreak, Snow marked cholera deaths around Broad Street in Soho and showed that most were clustered near one public water pump. Officials removed the pump handle, but the worst of the outbreak had already passed, so the map mattered more for the proof than for the cure.

Words to know

• sphincters: ring-shaped muscles
• microbiome: gut bacteria community
• reabsorption: taking back in
• ligands: binding molecules
• emulsify: break into droplets