Most burps are not mysterious gas being manufactured by the stomach.
They are mostly air that entered the digestive system from the outside.
You swallow small amounts of air throughout the day.
Eventually some of it comes back.
We swallow air while eating and drinking
Every time you eat or drink, a little air can go down with the food or liquid.
Eating quickly, drinking through a straw, chewing gum, talking while eating, or gulping can increase the amount.
That swallowed air can collect in the upper stomach.
Carbonated drinks add extra gas
Soda and other fizzy drinks contain dissolved carbon dioxide.
When the drink warms and pressure changes inside the stomach, some of that gas comes out of solution.
Now there is more gas available to move upward.
That is why a carbonated drink can produce a rapid series of burps.
How does the gas get back out?
A muscular ring between the esophagus and stomach normally helps keep stomach contents from moving upward.
When gas pressure builds, that area can relax briefly.
Gas travels into the esophagus and then out through the mouth.
That is a belch or burp.
Why do some people burp more than others?
Eating habits matter.
So do carbonated drinks, gum chewing, anxiety, and the amount of air a person unconsciously swallows.
Digestive conditions can affect burping too.
Occasional burping is completely normal.
Is a burp the same as a fart going the wrong direction?
No.
A burp mostly involves swallowed air or gas in the upper digestive tract.
Intestinal gas is produced farther down, often by bacteria fermenting food in the large intestine.
The two exits are handling somewhat different inventories.
Why can burps smell?
If gas carries odors from recently eaten food or stomach contents, a burp can have a noticeable smell.
But many ordinary burps are mostly odorless air.
So a burp is not usually your stomach creating some strange internal weather event.
It is mostly returning air you accidentally mailed there earlier.
A related explanation is Why do we fart?, which looks at another familiar effect from the same general system.
And that’s the short explanation.
Related explanations: Why do we fart?, Why does soda fizz?, and Why do we get hiccups?