Pee and poop are two ways the body lets go of what it cannot use. Pee is waste and extra water that your body does not need. 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.

The trip starts with chewing. Digestion begins in your mouth, and spit makes food easy to swallow. Once you begin swallowing, the process becomes automatic, so you do not have to think about it. 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.

The stomach makes acid that helps break food down. Protein digestion begins here, where acid and an enzyme called pepsin break it down. The stomach also churns food with its juices into a thick mix called chyme. Stomach enzymes that digest protein could digest the stomach itself, so a thick coat of bicarbonate-rich mucus forms a barrier that neutralizes acid.

The liver makes bile, a digestive juice that helps break down fats, and the gallbladder stores it. Bile acts like dish soap on grease: its salts break fats into tiny droplets (emulsify them) so the body can digest and absorb them. Bile also colors the end of the trip. The liver sends a yellow substance called bilirubin out in bile, and bilirubin helps give poop its brown color. Poop is usually some shade of brown.

The small intestine takes nutrients from digested food into the blood, and it takes in water as well. Tiny bumps called villi, about 20 to 40 in every square millimeter, give it a huge surface for taking in nutrients. The pancreas helps by making more than a liter of digestive juice a day, with enzymes that break down carbohydrates, fats and proteins.

Bacteria that live in the gut help digest food. 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 large intestine takes water out of leftover food and turns it into solid poop. Poop is more than leftover food: the waste that reaches the large intestine also includes fluid and older cells from the lining of the gut. The rectum, the last part of the large intestine, holds poop until it is time to go. When it is full, nerves tell you that you need to poop. Two ring-shaped muscles called anal sphincters keep the anus closed, and you can choose when to open the outer one.

You have two kidneys, one on each side of your back, just under your ribs. 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. Pee is made of water, salts and waste that were filtered out of the blood.

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).

Each day the kidneys filter well over a hundred quarts of fluid out of the blood, but about 99 percent of it goes back into the blood, and only 1 to 2 quarts become pee. Kidneys do more than make urine. They remove acid made by the body's cells, and they keep the water, salts and minerals in the blood in balance so nerves and muscles work normally. They also make hormones that help control blood pressure, build red blood cells and keep bones strong.

The bladder is a hollow, muscular, balloon-shaped organ that acts like a reservoir and can hold about 1.5 to 2 cups of urine. Two sets of sphincter muscles squeeze the exit tube shut, and the inner ones stay closed until the brain sends a signal to urinate. Peeing is a teamwork job between brain and muscle. 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. The cube shape forms inside the intestine, not at the anus. 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.

In 1849 the London doctor John Snow published a paper, On the Mode of Communication of Cholera. He argued that cholera spread through contaminated water, not through bad air, which went against what most doctors of his day believed. Snow found the Broad Street pump partly by talking to people. With help from a local clergyman, Reverend Henry Whitehead, he interviewed residents of the neighborhood and identified the public pump on Broad Street as the source. Most people who died had drunk from it, and later it was found that a leaking sewer ran near the well. Snow's work is a model of how we know. He used a map of deaths, interviews, and a comparison of two water companies. One of these comparisons has been described as a natural experiment, because the water companies, not the researchers, decided who got which water.

Words to know

• cholera: water-borne illness
• emulsify: break into droplets
• microbiome: gut bacteria community
• glomerulus: kidney filter
• reabsorption: taking back in