Everybody has gas, and everybody burps and farts. Burping and passing gas are natural and common. They are normal parts of how a body works. A burp is air from inside you coming back up and out through your mouth. A fart is gas from inside your belly that comes out of your bottom. So where does all that gas come from? It has two sources, and one of them is a surprise.

The first source is air. You swallow a little air every time you eat and drink. You swallow extra air when you eat or drink fast, or when you chew gum. Talking while you eat can do it too. Most of this air takes the quick way out. Most swallowed air comes back up as burps, and very little of it goes on to the rest of the digestive system.

Fizzy drinks are a good example. They have gas bubbles in them, and those bubbles can come back up as burps. Some swallowed air does skip the burp. A little of it moves on into the intestines. But most of that small amount is soaked up into the blood instead of coming out. People burp up to about 30 times a day, and burps come most often soon after eating.

The second source is the surprise. Tiny helpful germs called bacteria live in your large intestine. They break down food your body could not digest, and that makes gas. Most of the gas that comes out of the bottom is made this way, by bacteria, and not swallowed. So a fart is partly a sign of tiny living helpers at work.

Scientists have a word for what the bacteria do. They ferment food. That means they break down food that was not digested earlier and give off gas as a byproduct. The bacteria always make some gas. The longer food stays in the large intestine, the more time the bacteria have to ferment it, so more gas can build up.

Which food gets to the bacteria? Food carbohydrates come as sugars, starches and fiber. The stomach and small intestine cannot fully digest some of them, so they travel on to the large intestine. Beans are famous for this. Foods with fiber, like beans and cabbage, make more gas because the body cannot fully digest fiber, so bacteria in the large intestine do the job.

Milk can do something similar for some people. Some people have extra gas after milk, cheese or ice cream, because their bodies have trouble digesting the sugar in milk, called lactose. Two things change how much gas a person has: how much air they swallow, and which foods they eat. That is why two people at the same table can have very different afternoons.

Now for the smell. You may have heard that farts are mostly stinky. In fact, most gas has no smell at all. Gas is a mix of gases: carbon dioxide, nitrogen, oxygen and hydrogen, and sometimes methane. Most of these have no smell. A fart may smell or may not. When it does smell, sulfur is often the reason.

A small part of fart gas is hydrogen sulfide. It smells like rotten eggs and is made by the microbes in the large intestine. The stinky part of a fart comes from gases that have sulfur in them.

How much is normal? Most people pass gas up to about 20 times a day, and they pass it more often around mealtimes and just after eating. What counts as a normal amount is different for every person. Some people pass gas more than others, and that is normal. Your body is a busy place, and the gas is just one sign that it is working.

Words to know

• bacteria: tiny living things
• ferment: break down food
• fiber: plant food parts
• lactose: milk sugar
• sulfur: smelly mineral