Poop is the leftover parts of food that your body cannot use. Pee is waste and extra water that your body does not need. Food goes in at your mouth and poop comes out at your anus, at the other end of one long tube. Digestion starts in your mouth when you chew, and spit makes food easy to swallow. Your stomach makes acid that helps break food down. Muscles in the walls of your gut squeeze food and poop along, and the squeezing is called peristalsis. So what you cannot use ends up as poop after a long trip down the tube.

In the stomach, food is churned with the stomach's juices into a thick mix called chyme. Protein digestion begins in the stomach, where acid and an enzyme called pepsin break it down. Stomach enzymes that digest protein could digest the stomach itself, so a thick coat of bicarbonate-rich mucus forms a barrier that neutralizes acid. Without that coat, the acid could harm the stomach wall. The stomach 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 it is usually empty within 2 to 4 hours.

The small intestine takes nutrients from digested food into the blood. Tiny bumps called villi, about 20 to 40 in every square millimeter, give the small intestine a huge surface for taking in nutrients. Fats need help from the liver. The liver makes roughly half a liter to a liter of bile a day, and the gallbladder stores it between meals, so it is ready when a meal comes. Bile acts like dish soap on grease: its salts break fats into tiny droplets, which is called emulsifying them, so the body can digest and absorb them.

Whatever the body does not take in moves on to the large intestine. 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. The large intestine also takes water out of leftover food and turns it into solid poop, so the soft leftovers become firm. Poop is usually some shade of brown. The liver sends a yellow substance called bilirubin out in bile, and bilirubin helps give poop its brown color. Bile, a fluid made by your liver, gives poop its brown color.

The rectum is the last part of the large intestine. It holds poop until it is time to go, and 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. Your body makes pee and poop on its own, but you get to choose when to go to the bathroom. That gives you time to reach a bathroom. The rectum fills, the nerves send their message, and the outer muscle stays closed until you open it.

Wombats are the only animals known to make poop shaped like cubes, as of November 2026. 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 cubes get their flat sides and corners as stiff and soft parts of the gut squeeze and relax. The cube shape forms inside the intestine, not at the anus.

Your kidneys clean your blood, taking out waste and extra water to make pee. You have two of them, one on each side of your back, just under your ribs, and each kidney is about the size of a fist. Each kidney has about a million tiny filtering units called nephrons. The kidney filter lets water and small wastes through but keeps blood cells and large proteins in the blood; it sorts by particle size, so small things pass through and large things stay behind. Nephrons take a simple filtrate of the blood and change it into urine.

Pale yellow pee usually means you have plenty of water in your body. Dark yellow pee usually means there is less water in it, and you may need a drink. The yellow color in pee comes from a pigment called urochrome, also called urobilin. The color of pee depends on how much urobilin is in it and how much water is in it. When you are dehydrated, pee is darker and more concentrated and may smell of ammonia, which comes from urea. The color and smell of pee are rough clues to how much water your body has.

Two thin tubes called ureters carry pee from the kidneys to the bladder, which is like a stretchy balloon that gets bigger as it fills with pee. Stretch receptors in the bladder wall send nerve signals to the brain as the bladder fills. Two sets of sphincter muscles hold the bladder closed: the inner one works automatically, and you can squeeze the outer one on purpose. If anything ever hurts when you go to the toilet, tell a grown-up. Wetting the bed or having an accident is common in children and is almost never caused by laziness or poor toilet training.

Words to know

• emulsifying: breaking up fat
• villi: tiny gut bumps
• nephrons: tiny kidney filters
• bilirubin: yellow bile pigment
• urobilin: yellow pee pigment