Throwing up feels bad, so why does the body do it at all, and which animals can do it too? As of November 2026, scientists describe nausea and throwing up as protective defence processes that help humans and other animals able to vomit avoid taking in or digesting poisons. Not every animal can do it, and most animals are non-emetic, which means they cannot throw up, and mice and rats are two examples, while dogs, cats, ferrets and shrews can. Kids often throw up at some point, and the spell usually ends soon. Throwing up can be the way a body protects you by getting rid of something that might hurt it.

Let's meet some things that can start the signal. Food that has gone bad can cause food poisoning, and that can make you throw up. Germs cause stomach bugs, which are the most common reason kids throw up. These germs spread from person to person, and soap and water on your hands help keep them from spreading. A day or two is usually all it takes for the throwing up from a stomach bug to stop. Salivation, or extra spit, can come just before you throw up. Babies often spit up small drops of milk. Throwing up, on the other hand, takes strong muscle squeezes. Each of these is a different way for the signal to start.

An episode of throwing up runs in three phases. At first, in pre-ejection, you may have nausea, salivation, swallowing, pale skin and a faster heartbeat. Next, in ejection, there is retching and throwing up, and last, in post-ejection, the body settles back down. Vomit usually has the look of the recent meal, chunks of food and all. Bile, when it is present, tastes bitter and looks yellow-green. Afterward, many people feel better for a time, because nothing that bothered the stomach is left in it. Spit can pool in your mouth during that first step. The food pipe carries food down to the stomach, and in throwing up it carries food back up the same way. Feeling sick can leave a person pale and sweaty too.

The middle phase is retching. Retching is a rhythmic squeezing by the diaphragm, chest wall and belly muscles, with the mouth and glottis kept closed. The stomach contents slosh between the stomach and the esophagus. Scientists split emesis, another word for vomiting, into three separate parts of the digestive tract. The first part is the stomach and throat set-up. The second is a backward giant contraction, and the third is the retching and expulsion actions. Each part has its own brainstem network, and one central pattern generator ties the three together. At the foot of the esophagus sits a ring of muscle that acts as a door: it opens for food heading into the stomach, then squeezes shut so food and acid do not flow back up.

Next, who is in charge of all of this? A group of brainstem structures called the dorsal vagal complex controls the throwing-up reflex, and it is made mostly of the area postrema and the nucleus tractus solitarius. The area postrema is a poison alarm of sorts, since it senses chemicals in the blood, and the dorsal vagal complex's inputs include signals from the gut's vagus nerve, chemicals in the blood, the balance nuclei in the brainstem and the insular cortex. These structures sit in the brainstem, which is the part of the brain that helps with the decision.

Messages reach the brainstem's vomiting center from the gut, the throat, the inner ear and the thinking parts of the brain, and the gut is the first source. The vagus nerve runs a long way, from the brain through the neck to the body's organs, and signals that set off heaves can travel along it. More than 90 percent of the body's serotonin is made by cells in the gut lining. This chemical is key to gut movement, nausea and throwing up. The throat is the next source. A gag reflex guards the throat against choking when something goes too far down it.

The inner ear and the thinking parts of the brain are the last two sources. The inner ear's balance parts connect to the part of the brain that controls throwing up. Big feelings, from fear and disgust to worry and even big excitement, can bring on throwing up in some people. Adults and children use the same steps, though a kid's reflex is easier to set off. A kid's body is more sensitive than a grown-up's and can react to things that would not bother an adult. Each of these sources can send its own message. Eating too much can stretch the stomach too full, and kids sometimes throw up after that.

How do scientists know which part does what? Chemicals in the blood can turn on the chemoreceptor trigger zone of the brainstem. Yet when that zone is stimulated directly with electricity, an animal does not vomit. This means the trigger zone is just one part of the circuit. Some chemicals in the blood can set off the urge to throw up through this zone. The area postrema, the poison alarm, also senses chemicals in the blood, and the brainstem has a say in when you throw up. A test like this shows that one part alone does not do the whole job.

Motion sickness is a puzzle. As of November 2026, there are two main ideas about it. The first idea, sensory conflict, says the senses disagree. The second, postural control, says the trouble is an inability to adjust posture, and it has little evidence behind it. The first idea says the eyes, inner ears and body report movement that clashes with what the brain expects. Motion sickness can happen when the eyes and the inner ears send the brain different messages about how the body is moving. Some people get dizzy and sick to the stomach on a car or boat trip. Each idea tries to explain the same thing. Both ideas are about why motion makes some people feel sick.

Cars give one clue. Boats and planes can make some people feel sick too. Many people feel motion sick as passengers but not as drivers. This is probably because drivers are better at anticipating the vehicle's motion and move with it. Keeping an eye on the road ahead or the horizon can ease the feeling, since the eyes then see the motion that the inner ears feel. When the eyes and the ears agree, the car-sick feeling can ease. 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

• emesis: vomiting
• retching: rhythmic squeezing
• serotonin: gut chemical
• brainstem: base of brain
• bile: bitter fluid