Air that took a wrong turn
Where Does a Burp Come From?
Almost every burp is air you swallowed without noticing, on its way back out of the only door it came in by.
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Step 1 of 5
Every swallow takes air with it.
A mouthful of food or drink traps a bubble of air behind it. Talking while eating, drinking fast, or chewing gum all swallow far more.
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Step 2 of 5
Gas floats, so it collects at the top.
Stomach contents are mostly liquid, and gas is far less dense, so any air separates out and gathers under the upper wall within seconds.
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Step 3 of 5
It cannot carry on downward.
The exit at the bottom of your stomach is under the liquid, so gas never reaches it. The only opening the air can reach is the one it came in through.
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Step 4 of 5
A ring of muscle lets it out.
The valve at the top of the stomach is normally shut tight. When pressure builds it relaxes for a fraction of a second, and the escaping gas makes the noise.
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Step 5 of 5
Fizzy drinks skip the swallowing entirely.
A can holds several times its own volume in dissolved gas. Warmed in your stomach, it comes out of solution all at once and has to go somewhere.
A 330 ml can of fizzy drink holds around 800 ml of carbon dioxide dissolved in it — more than twice the volume of the drink itself.
The short version
A burp is swallowed or fizzed-out gas floating to the top of your stomach, where the only opening it can reach is the way it came in.
Try it yourself
Drink water through a straw versus gulping from the glass. The straw takes in far less air, and far fewer burps follow.
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