Poop and pee are different from each other. 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. Muscles in the walls of your gut squeeze food and poop along, and the squeezing is called peristalsis. The small intestine takes nutrients from digested food into the blood, and it takes in water as well as nutrients.

The large intestine takes water out of leftover food and turns it into solid poop. The rectum is the last part of the large intestine, and it holds poop until it is time to go. When the rectum 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. That is how your body can wait until you find a toilet.

Now follow the pee. 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 it comes out cleaned through the renal vein. So the blood that goes in carries waste, and the blood that comes out has been cleaned. A fist is about as big as your hand closed up tight.

Inside the kidney, the work is done by very small parts. Each kidney has about a million tiny filtering units called nephrons. A million is a very big number, and each of these units is tiny. Each nephron has a filter called the glomerulus. Pee is made of water, salts and waste that were filtered out of the blood. Adults make about 1 to 2 liters, or 1 to 2 quarts, of pee a day.

The yellow color in pee comes from a pigment called urochrome, also called urobilin. The color depends on how much urobilin is in it and how much water is in it. 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 color and smell of pee are rough clues to how much water your body has. So a quick look at the color can give you a hint.

Two thin tubes called ureters carry pee from the kidneys to the bladder. Your bladder is like a stretchy balloon that gets bigger as it fills, and an adult bladder can hold about 1.5 to 2 cups of pee. When your bladder gets full, it sends a message to your brain that you need to find a toilet. To pee, the bladder wall squeezes and pushes urine through the urethra and out of the body. The urethra is the tube that carries it out.

Your body makes pee and poop on its own, but you get to choose when to go to the bathroom. Most children learn to control their bladder between ages 2 and 4, each in their own time, so nobody needs to feel bad about it. Wetting the bed or having an accident is common in children, and it is almost never caused by laziness or poor toilet training. If anything ever hurts when you go to the toilet, tell a grown-up right away.

Toilets have a long history. More than 4,500 years ago, houses in the city of Mohenjo-daro, in today's Pakistan, had toilets connected to drains. The Romans built complex sewers that carried waste away, usually to nearby rivers and streams. In the Middle Ages, castles had shafts for emptying waste, and many town homes used cesspits. Some of these used drains, some used sewers, and some used shafts or pits. In 1596 Sir John Harrington described a water-operated toilet, called a water closet.

Words to know

• nephrons: tiny kidney filters
• glomerulus: a nephron's filter
• sphincters: ring-shaped muscles
• ureters: kidney-bladder tubes
• cesspits: holes for waste