Burps and farts both start with gas, but they do not start in the same place. 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. Both are ways the body lets gas go, and each has its own story.

Start with burps. You swallow a little air every time you eat and drink. Eating or drinking fast makes you swallow extra air, and so does chewing gum. Talking while you eat can do it too.

Fizzy drinks add more. They have gas bubbles in them, and those bubbles can come back up as burps. People burp up to about 30 times a day, and burps come most often soon after eating. Where does swallowed air go? That is a fair question to ask. Most of it comes back up as burps. A little can skip the burp and move on into the intestines, but most of that small amount is soaked up into the blood instead of coming out.

Now for farts. You might guess that swallowed air makes them. But most of the gas that comes out of the bottom is made by bacteria in the large intestine as they break down food, not swallowed.

Why is there food left over for the bacteria to use? 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.

The bacteria always make some gas. When there is more undigested carbohydrate for them to work on, there tends to be more gas. Foods with fiber, like beans and cabbage, make more gas. The body cannot fully digest fiber, so the bacteria do the job.

Milk can do it too. Some people have extra gas after milk, cheese or ice cream, because their bodies have trouble digesting the sugar in milk, called lactose.

Gas is a mix of gases. It has carbon dioxide, nitrogen, oxygen and hydrogen, and sometimes methane. Most of these have no smell. That is why a fart may smell or may not.

When a fart does smell, sulfur is often the reason. Some foods, such as cabbage and meat, can make gas smell stronger. So what you ate can change the smell, even though the gas itself is made the same way.

Most people pass gas up to about 20 times a day. Burping and passing gas are natural and common, and they are normal parts of how a body works. Everybody has gas, and now you know where it comes from.

Words to know

• bacteria: tiny living things
• fiber: plant food bulk
• carbohydrates: sugars and starches
• digest: break down food

The short version

A burp is swallowed air coming back up through the mouth. A fart is gas made mostly by bacteria in the large intestine as they break down food. Both are normal. Most people pass gas up to about 20 times a day, and burp up to about 30.

Quick facts

• A burp is air from inside you coming back up and out through your mouth.
• Most of the gas that comes out of the bottom is made by bacteria in the large intestine.
• People burp up to about 30 times a day, most often soon after eating.
• Most people pass gas up to about 20 times a day.
• A small part of fart gas is hydrogen sulfide, which smells like rotten eggs.