The Science of Flatulence: A B1 English Reading Unit

Level: B1

Printable: Student handout (PDF) | Teacher’s key (PDF)

Reading

Flatulence is the medical word for passing gas from the digestive system. People also call it farting. Everyone does it, and doctors say it is normal and not dangerous. Even so, many people find the topic embarrassing. In this lesson, we look at the science: where the gas comes from, how much people pass, and how animals compare.

Gas in the gut has two main sources. The first is air that we swallow when we eat or drink quickly, chew gum, or drink fizzy drinks. Most of this air comes back up as a burp. The second source is larger: bacteria in the large intestine digest food that our own bodies cannot break down, especially fiber and some sugars. As the bacteria work, they make gases. In one study of 16 people, about three quarters of the gas passed was made inside the intestine in this way.

Most of the gas has no smell. It is mostly nitrogen, hydrogen, carbon dioxide, and sometimes methane. Only some people make methane, because it depends on which microbes (tiny living things) live in their gut. The bad smell comes from tiny traces of gases with sulfur in them. In a 1998 US study, the stronger the smell of a sample, the more hydrogen sulfide it had.

The gas moves through the intestines and collects near the end of the digestive system. The muscles around the anus hold it in, and most of the time we can choose when to let it out. In a 1991 British experiment, ten volunteers ate their usual food plus 200 grams of baked beans. Their daily amount of gas was between 476 and 1,491 milliliters, and the middle value was 705 milliliters, about two soda cans. Larger amounts were passed after meals than at other times. On a fiber-free diet, the amount fell to about 214 milliliters a day.

How often do people pass gas? In a 1996 US study, 25 healthy adults counted each time for a week. The average was 10 times a day, and 20 was the top of the normal range. The US National Institutes of Health gives an average of 8 to 14 times a day and says up to 25 can be normal. In a survey of 16,000 Americans, the most common answer was passing gas about every three to four hours, and people aged 65 and over reported it less often than younger adults. These answers were self-reported. But a 2025 study at the University of Maryland in the US found much higher numbers. Healthy young adults wore a small hydrogen sensor on their underwear. It recorded an average of 32 times a day, from 4 to 59. The team says older counts may be too low because people forget or do not notice.

Many animals make gas too. Cows make a lot of methane, but most of it leaves through the mouth as burps, not from the other end. Herring make short, high-pitched sounds at night, and scientists think the fish use them to communicate. This is not digestive gas, however: herring gulp air at the surface and let out bubbles near the anus. Scientists have even estimated that giant plant-eating dinosaurs made huge amounts of methane, although nobody can be sure.

Passing gas is a natural part of digestion, and eating more slowly means less swallowed air. People who have strong pain or other worrying symptoms with gas should talk to a doctor. The Maryland team now plans to measure hundreds of people with its sensor. That may finally show how much gas is really normal.

Vocabulary

Each word has a frequency score, a rank, and a tag. The score shows how common the word is in modern English: higher means more common, and each full point means roughly ten times more common. The rank is the word’s position on a list of about 321,000 English word forms, from most to least common. The tag shows where you are most likely to meet the word: Current, Formal, Literary, or Dated. The paragraph number shows where the word first appears in the reading. The labels are information, not instructions. Deciding which words are worth your time is up to you. How to read the scores, ranks, and tags.

Doctors say passing gas is normal and not dangerous.
It is normal to feel nervous before a test.

Even so, many people find the topic embarrassing.
Forgetting my friend’s name was embarrassing.

We swallow air when we eat or drink quickly.
It hurts to swallow when you have a sore throat.

Bacteria in the large intestine digest food.
Some bacteria are harmful, but many are useful.

Bacteria digest food that our bodies cannot break down.
Milk is hard to digest for some people.

Beans and vegetables contain a lot of fiber.
Bacteria digest fiber and some sugars.

Most of the gas has no smell.
I love the smell of fresh bread.

The muscles around the anus hold it in.
Swimming makes your muscles stronger.

Ten volunteers ate their usual food plus 200 grams of baked beans.
The city needs volunteers to clean the beach on Saturday.

In the study, the average was 10 times a day.
The average age of the students is twenty.

Twenty was the top of the normal range.
The shop sells a wide range of bicycles.

Their daily amount of gas was between 476 and 1,491 milliliters.
A small amount of salt is enough.

In a survey of 16,000 Americans, the most common answer was every three to four hours.
The school did a survey about students’ sleep.

Scientists think herring use the sounds to communicate.
Whales communicate with long songs.

Scientists have estimated that giant dinosaurs made huge amounts of methane.
The builder estimated that the job would take three weeks.

Activity Page

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Speed Challenge

Answer 12 questions against the clock. Every wrong answer adds 5 seconds, and the lowest score wins. Open the speed challenge in a new tab.

Comprehension Questions

What is flatulence, and what do doctors say about it?
Explain the two main sources of gas in the gut. Which source is larger?
What do the bacteria in the large intestine do, and what kinds of food do they use?
Which gases are in the gas that people pass? What gives some of it a bad smell?
What do the muscles around the anus do?
Describe the 1991 experiment. Who took part, what did they eat, and what did the researchers find?
What do the numbers in the reading show about how often people pass gas? Which study found the highest number, and how did it count?
What did the survey of 16,000 Americans find, and why might answers like these not be exact?
Compare cows and herring. In what way is the herring case different from normal flatulence?

IELTS-Style Questions

True, False, or Not Given: “Beans always make people pass gas.” Explain your answer using evidence from the reading.
Yes, No, or Not Given: “The writer thinks that people should feel embarrassed about passing gas.” Explain your answer using evidence from the reading.
Author’s purpose: Why does the writer include cows, herring, and dinosaurs in the last part? What do they add to the lesson?

AI Discussion Prompt

I just read a B1 English lesson called The Science of Flatulence. It explained that gut gas comes from swallowed air and from bacteria that digest food, that most gas has no smell but sulfur gases cause the bad smell, that older studies counted about 8 to 14 times a day while a 2025 sensor study recorded about 32, and that animals such as cows and herring also let out gas or bubbles. I am an English learner at about B1 level. Please talk with me about digestion and food in a friendly, polite way. Use simple English and ask one question at a time. After my answer, gently correct one or two mistakes and show me a better way to say it.

AI Vocabulary Prompt

I want to practice vocabulary from a lesson about the science of flatulence. Please quiz me on the following words one at a time, using a mix of question types: multiple choice, true/false, and sentence completion. Use simple English. Give me feedback after each answer before moving on, and bring back the words I miss later. Here are the words: normal, embarrassing, swallow, bacteria, digest, fiber, smell, muscles, volunteers, average, range, amount, survey, communicate, estimated.

Discussion Questions

Before you read this lesson, what did you know about where gut gas comes from? What was new?
Why do you think many people find the topic embarrassing, even though it is normal?
The reading says bacteria in our gut help us digest food. Did that surprise you? Why or why not?
Which foods do you think make more gas? How could a scientist test your idea in a safe and fair way?
The study used volunteers who ate baked beans. Why might a scientist use the same food for every volunteer?
The results show a wide range between people. Why is it important to report the range and not only the average?
The survey of 16,000 people used self-reported answers. What problems can self-reported answers have?
Scientists plan to measure hundreds of people with the new sensor. What other everyday body activity would you like scientists to measure, and why?
Cows make a lot of methane, mostly from burps. Why might this matter for the environment?
Herring seem to use bubbles to communicate. What other animals communicate without using their voices?
Some scientists estimated that dinosaurs made huge amounts of methane, but nobody can be sure. How can scientists learn about animals that lived millions of years ago?
The lesson says you should talk to a doctor if you have strong pain. Why is it a good idea to ask a doctor about health worries and not only search online?
Eating more slowly can reduce swallowed air. What other simple habits help our digestion?
How would you explain the science of digestion to a younger student in three sentences?

Language Focus: Comparatives and Superlatives

What it is: We use comparatives (more, less, larger, -er) to compare two things or groups, often followed by than. We use superlatives (the most, the largest, the -est) to say which is at the top of a group of three or more. The pattern the + comparative, the + comparative shows that two things change together.

Why writers use it: Science texts compare amounts, groups, and results all the time. Comparatives and superlatives help you say how big, how common, or how often something is.

Examples from the reading:

“The second source is larger.”
“Larger amounts were passed after meals than at other times…”
“…the most common answer was passing gas about every three to four hours…”
“…people aged 65 and over reported it less often than younger adults.”
“…the stronger the smell of a sample, the more hydrogen sulfide it had.”

Practice task: Complete the sentence with a comparative: “The amount of gas on a fiber-free diet was ____ (small) the amount on a normal diet.”

(Example answer: “The amount of gas on a fiber-free diet was smaller than the amount on a normal diet.”)

Recommended Resources

Symptoms and Causes of Gas in the Digestive Tract (US National Institute of Diabetes and Digestive and Kidney Diseases). A clear health page that explains where gas comes from, how often people pass it, and what can cause extra gas.
‘Smart Underwear’ Measures How Often Humans Fart (Science News). A news story about the sensor study that counted about 32 gas events a day, and why scientists want better numbers.
Identification of Gases Responsible for the Odour of Human Flatus (Suarez, Springfield, and Levitt, Gut, 1998). The abstract of the study that linked bad smell to sulfur gases, especially hydrogen sulfide. Read the results section first.
Pacific and Atlantic Herring Produce Burst Pulse Sounds (Wilson, Batty, and Dill, Biology Letters, 2004). The short scientific paper on the herring sounds. It is difficult, but the abstract is only a few sentences.
Which Is a Bigger Methane Source: Cow Belching or Cow Flatulence? (NASA Science). A short answer from NASA about where cow methane comes from.