A fart is gas from inside you that comes out of your bottom. But gas is not just one thing. It is a mix of gases, such as nitrogen and oxygen. Most of these gases have no smell, so a fart does not always smell bad.

So why does a fart sometimes smell bad? A small part of fart gas is hydrogen sulfide. It smells like rotten eggs. The bad smell comes from gases that have sulfur in them. Microbes in the large intestine make this gas, as you will see next.

Microbes are tiny living things that help you. They live in the large intestine. They break down food that your body could not digest, and that makes gas. Some foods make more gas than others, and beans are one of them.

Some people get extra gas after milk, cheese or ice cream. Their bodies have trouble with the sugar in milk, called lactose. Most people pass gas up to about 20 times a day, and everybody does it. It is nothing to be shy about.

Now think about burps. You swallow a little air every time you eat and drink. You swallow extra if you eat fast, chew gum or talk while you eat. Drinks with bubbles have gas bubbles that can come back up as a burp.

Most swallowed air comes back up as burps. Very little of it goes on to the rest of the digestive system. Gas leaves the body in two ways. A burp and a fart are both normal parts of how a body works.

Words to know

• sulfur: smelly stuff
• microbes: tiny living things
• lactose: milk sugar
• swallow: gulp down