Vomiting is another name for throwing up. In throwing up, the stomach pushes its contents back up and out of the mouth. Kids throw up now and then, and it is usually over quickly. Kids throw up more easily than grown-ups, because their bodies are more sensitive and can react to things that would not bother an adult. So what is your body doing when it happens? Let's take it one part at a time, starting with the stomach.

The first part of the story is the stomach. Digestion starts in your mouth, where spit begins to break down some of your food. The stomach is an organ that sits in your belly. The muscles of the stomach churn and mix the food that you eat. It also makes acid that helps break food into smaller pieces. Acid sounds rough on a soft belly, but your stomach has a tough lining. Part of the stomach makes mucus, a slippery coat, that protects the lining from digestive juices.

Your body gives you warnings first. Often you feel sick to your stomach first. That feeling is called nausea. Nausea is the feeling of wanting to vomit. Extra spit, called saliva, can fill your mouth right before you throw up. You may also go pale and feel sweaty. Then the squeeze begins. The strong muscles of your belly wall squeeze the stomach, and the diaphragm pushes down hard. Just before vomit comes up, the glottis, the opening to your windpipe, shuts. At the same moment the larynx, your voice box, rises.

Where does it all go? Food goes down a tube called the esophagus, or food pipe, to your stomach. When you throw up, the food goes back up that same tube. At the bottom of the esophagus, a ring of muscle works like a door. The door opens to let food into the stomach, and then it squeezes shut, so food and acid stay down. Your body also has a gag reflex that helps protect you from choking, and something that goes too far down your throat can set it off.

Who tells the body to do all this? Your brainstem is a part of the brain, and it helps decide when you throw up. A long nerve called the vagus nerve runs from the brain through the neck to the body's organs. It can carry signals that set off heaves and throwing up. Another part of the brain, called the area postrema, acts like a poison alarm. It senses chemicals in the blood and can start the urge to vomit.

Throwing up comes in different sizes. Small spit-ups of milk are common in babies. Throwing up is not the same, because it takes strong muscle squeezes. Eating too much can make a kid throw up, because the stomach has been stretched too full. The worst vomiting can shoot out of the mouth so hard that it goes more than a metre, and this is called projectile vomiting. Many parts of the body work together in this way.

Some people get sick from a ride. A ride in a car or a boat can leave some people dizzy and sick to their stomachs. Motion sickness can come when your eyes and inner ears give your brain different messages about how you are moving. Gazing at the road ahead or the horizon can ease the car-sick feeling, because your eyes then see the same motion that your inner ears feel.

Your body needs some care afterward. Losing a lot of fluid by vomiting can leave the body short of water. That is called dehydration, and water and tiny minerals called electrolytes are lost in the vomit. Stomach acid can soften the outer layer of your teeth, called enamel. After you throw up, drinking or rinsing with water helps protect them. And if you feel sick, tell a grown-up, like your mom or dad. A grown-up can check on you and get a doctor if you need one.

Words to know

• nausea: feeling sick
• esophagus: food pipe
• mucus: slippery coat
• dehydration: short of water
• enamel: outer tooth layer