You swallow gas (air) as you eat and other gas is formed in your body as it digests food, but healthy people and animals always have gas in their digestion tracks. If the gas comes back up from the stomach, we call it a burp. If it comes out after digestion, we call it a fart or flatulence or passing gas. What you eat and how much you eat changes what the gas smells like and how much gas a body will produce.