Lung Model Bottle Experiment: Build One and Breathe

Pull the stretched balloon at the bottom of the bottle and the balloon inside puffs up. Push it back and the inner balloon collapses. Nobody blew into anything. That is the whole idea the lung model bottle experiment delivers: you do not suck air into your chest. You make your chest bigger, the pressure inside drops, and the atmosphere pushes air in. Most children, and plenty of adults, believe lungs are muscles that pull. They aren’t. They’re passive bags, and the muscle doing the work sits underneath them.

Key Takeaways

• Cut the bottom off a 2 L bottle, hang a balloon inside from the neck, stretch a cut balloon over the open bottom, pull down.
• It teaches diaphragm mechanics and negative-pressure breathing: volume up, pressure down, air in.
• Fits ages 7 to 13. An adult does all the cutting.
• The one thing that goes wrong: air leaks. If the bottom membrane isn’t taped airtight, nothing happens at all.
• This model gets the diaphragm right and the ribcage completely wrong. Say so: it is better teaching than pretending the model is complete.

What You’ll Need

Item	How much	Where to get it	If you don’t have it
Clear plastic bottle, 1.5–2 L	1	Any fizzy drink bottle	A rigid-walled juice bottle. Avoid flimsy thin-walled water bottles — they collapse
Balloons	2	Party aisle, corner shop	Latex allergy: use nitrile or mylar balloons instead
Strong tape	~50 cm	Toolbox: electrical, duct or packing tape	Sticky tape doubled over, but it leaks more
Scissors or craft knife	1	Kitchen drawer	Adult only. Serrated knife works on a soft bottle
Rubber band	1	Drawer	A twist tie, or tape wrapped tight
Drinking straw (optional)	1	Kitchen	Only for the two-lung version

Time: prep 10 min, experiment 15 min, cleanup 5 min Cost: a packet of balloons, nothing else Mess level: low, but the cut bottle edge needs taping

Do the Experiment: The Lung Model Bottle Experiment

Step 1 — Adult cuts the bottle

Cut the bottom 4 to 5 cm off the bottle, all the way around. Cut plastic edges are genuinely sharp. Run a strip of tape around the cut rim, folded over the edge, before anyone else touches it. Children do not do this step.

Step 2 — Hang the inner lung

Push one balloon, neck first, down through the bottle’s mouth from the outside so the balloon body hangs inside the bottle. Fold the balloon’s lip back over the outside of the bottle’s mouth and secure it with the rubber band. Blow once through the bottle neck to check the balloon inflates inside.

Step 3 — Make the diaphragm

Cut the neck off the second balloon and cut the remaining body into a wide sheet or dome. Tie a small knot in the middle of the sheet, or tape a bottle cap to the centre, so there’s something to grip.

Step 4 — Seal the bottom

Stretch the balloon sheet over the open bottom of the bottle and tape it down all the way round, overlapping the tape so there are no gaps. This step decides whether the experiment works. Press around the rim with your thumb and look for puckers: every pucker is a leak.

Step 5 — Predict, then pull

Before pulling anything, ask: “If I pull this bottom sheet down, what happens to the balloon inside?” Record the answer. Then pull the knot down 2 to 3 cm, hold, and release. Repeat slowly five times.

Step 6 — Break it on purpose

Now peel back 2 cm of tape at the bottom, making a deliberate hole, and pull again. Nothing happens. That failure is the most instructive part of the build, and it is the model’s version of a punctured chest wall.

What you should see: the inner balloon inflates visibly, maybe to the size of a plum, when you pull the membrane down about 2 cm, and deflates when you let go. The movement is small and unglamorous: expect a balloon that gets slightly fatter, not one that fills the bottle. If it doesn’t move at all, it’s a leak, not a design flaw.

What’s Actually Going On

Breathing is pressure work. At rest, the air in your lungs sits at roughly atmospheric pressure. When the diaphragm, a dome of muscle slung under the lungs, contracts, it flattens and descends, and the volume of the thoracic cavity increases. Gas in a larger space at the same amount of gas means lower pressure. Air then flows from higher pressure outside to lower pressure inside, down the gradient, until the pressures equalise. Nothing pulls. Nothing sucks. The atmosphere pushes.

In quiet breathing the diaphragm descends only about 1 to 2 cm, and in a deep breath several centimetres, as described in De Troyer and Boriek’s review of respiratory muscle mechanics. That small movement moves roughly half a litre of air per breath in an adult. Exhaling at rest takes no muscular effort at all: the diaphragm relaxes, the elastic recoil of lung tissue and chest wall does the work, and air flows back out.

Your model reproduces this faithfully. Pulling the bottom membrane increases the volume of the sealed bottle, pressure in the bottle drops below atmospheric, and air pushes in through the bottle neck into the balloon. The hole you cut in step 6 destroys the seal, so volume changes no longer produce pressure changes — which is, mechanically, what happens in a pneumothorax when air enters the space around a lung.

Now the honest part, and the reason this article is worth more than the usual version. The model gets the ribcage completely wrong. The bottle is a rigid box. A real chest is not. The intercostal muscles between your ribs lift and rotate the ribs outward and upward during inspiration, adding substantially to the volume change — Ratnovsky, Elad and Halpern’s account of respiratory muscle mechanics treats the rib cage and diaphragm as coupled pumps, not as a box with a floor. The model also omits the pleural space, the thin fluid layer that couples lung to chest wall so the lungs follow the wall’s movement instead of flopping around inside a cavity. And a balloon is nothing like lung tissue: real lungs are a branching tree ending in hundreds of millions of alveoli, whose collapse is prevented by surfactant rather than by rubber elasticity.

Say all of this to your child. Harrison and Treagust’s typology of school science models makes the point that students routinely treat a teaching model as a miniature of the real thing, and that naming a model’s limits is part of teaching it. “This shows the diaphragm correctly and the ribs not at all” is a more useful sentence than “this is how your lungs work”.

How to Explain It to Your Kid

Ages 5–8: Make the room bigger and air walks in

Tell them the air outside is always pushing, gently, on everything. The balloon inside the bottle has a door open to the outside. When you make the bottle bigger inside, there is more room, so the air walks in through the door. When you squash it, the air walks back out.

Have them do the pulling while you watch the balloon, then swap. Then ask them to put a hand on their own belly and take a big breath. The belly moves out. That is the diaphragm going down and pushing everything below it out of the way.

Ages 9–12: Volume, pressure, and one real number

Introduce diaphragm and the relationship between volume and pressure: bigger space, lower pressure, same amount of air. Then give the number, about half a litre of air per quiet breath in an adult, and have them check it. Breathe out normally into a balloon, tie it off, and measure its diameter. A 10 cm sphere is about half a litre.

Then the measurement that makes it an experiment: pull the membrane down 1 cm, 2 cm and 3 cm, and sketch the inner balloon at each. Record how far you pulled and how big it got. Two variables, one relationship.

Ages 13+: Where the model breaks, and why that matters

With a teenager, run the critique. Hand them the bottle and ask what parts of real breathing it cannot represent. Expect them to find the rigid wall. Push further: what about exhaling? In the model you push the membrane up, which implies muscular effort — but quiet exhalation is passive elastic recoil, so the model misrepresents that too.

Then the extension: build the two-lung version with a straw Y-piece through the cap and two small balloons, seal the cap with modelling clay, and compare. Squeeze one side of the bottle while pulling the membrane and watch the two “lungs” fill unequally. That is a crude model of why posture and injury change ventilation between lungs.

The question to ask: “If I made a tiny hole in the side of the bottle instead of the bottom, would the balloon still inflate when I pull? What if the hole were smaller than a pinprick?”

Why Teaching This Early Pays Off

The respiratory system arrives in school biology somewhere between ages 10 and 14, and it is usually taught as a labelled diagram: trachea, bronchi, alveoli, diaphragm. Labels are not a mechanism. A child who has built the bottle already knows why the diaphragm matters, which turns the diagram into a reminder instead of a memorisation task.

The deeper payoff is the pressure-gradient idea, which is the same idea behind drinking through a straw, siphons, syringes, vacuum cleaners, weather fronts and why your ears pop on a plane. Children who learn once that fluids move down pressure gradients rather than being “pulled” stop needing a new explanation for each case.

And there is a model-literacy payoff that is arguably the most valuable part. Smith, diSessa and Roschelle argued that students’ partly-wrong ideas are productive raw material rather than errors to eliminate. A bottle that gets the diaphragm right and the ribcage wrong is the perfect object for that lesson: the model is useful and incomplete, and a child who can say which is which has learned something about how science works, not just about lungs. One build will not change anyone’s grades, no evidence supports that claim, but it reliably replaces “lungs suck air in” with something true. For the research on why building beats watching, see our piece on hands-on STEM learning versus watching.

Before You Start: What Parents Should Know

Best age	7 to 13. Below seven the fine taping is frustrating and the payoff is too subtle to read.
Supervision	Adult required. The adult does every cut and tapes the rim before handing the bottle over.
Real hazards	Sharp cut plastic edges; uninflated and burst balloon pieces are a choking hazard for under-threes; latex allergy.
Don’t	Don’t let a child cut the bottle, don’t leave balloon scraps on the floor, and don’t substitute a glass bottle for the plastic one.

Two specifics. Uninflated balloons and balloon fragments are among the most common choking hazards for small children — keep the offcuts in a bag and bin them when you’re done, especially if there is a toddler in the house. And if anyone in the family has a latex allergy, use nitrile or mylar balloons; “a bit of latex” is not a safe substitution.

When It Doesn’t Work

• Nothing moves when you pull. A leak, nine times out of ten. Re-tape the bottom membrane with overlapping strips and check the rubber band at the neck. Test by taping over the bottle mouth and pulling: the membrane should feel springy and resist.
• The bottle crumples instead of the balloon inflating. The bottle walls are too thin. Switch to a rigid carbonated-drink bottle, which is built to hold pressure.
• The inner balloon stays inflated after you release. Its neck is folded too tightly over the bottle mouth, so air can’t escape. Loosen the rubber band slightly.
• The membrane tears while you pull. Over-stretched. Cut a larger sheet from a bigger balloon, or use a piece of a latex glove, which is tougher.
• It works but it’s boring. It genuinely is undramatic. Make it visible by taping a thin paper arrow to the inner balloon, or do the step-6 hole and let the failure be the finale.

Take It Further

Measure the relationship. Mark the membrane knot’s travel in 1 cm steps. At each step, photograph the inner balloon against graph paper taped behind the bottle and count squares to estimate its width. Plot pull distance against balloon width. It won’t be a straight line, and the reason, the balloon’s own elasticity rising as it stretches, is a real physics conversation.

Add a second lung and a blockage. Run a straw Y-piece through a drilled cap sealed with modelling clay, with a small balloon on each branch. Then pinch one straw branch partly shut and pull. The unblocked lung fills more. Record the difference, then connect it to what a doctor hears with a stethoscope when one airway is narrowed, which is the same logic your own pulse-before-and-after measurements rely on: a system’s output tells you about its plumbing.

What to Watch For Over the Next 3 Months

• Week 4: They correct someone. “You don’t suck air in, the air pushes in” is the sentence to listen for.
• Month 2: The pressure idea shows up elsewhere: a straw, a syringe, a plunger, ears popping in a lift or on a flight.
• Month 3: They start criticising models unprompted: “that diagram doesn’t show the ribs moving”. That habit is worth far more than the lung facts.

Frequently Asked Questions

Why doesn’t my bottle lung work at all?

Almost always a leak at the bottom membrane. Air entering around the tape means pulling the membrane no longer lowers the pressure inside. Re-tape with overlapping strips pressed firmly, then test by covering the bottle mouth with your thumb and pulling — a sealed bottle resists noticeably.

Does the model show how the ribs work?

No, and that’s its biggest limitation. The bottle is rigid, so it can only show the diaphragm. In real breathing the intercostal muscles lift and rotate the ribs outward, contributing a large share of the volume change. Tell your child the model is deliberately incomplete.

Are lungs really like balloons?

Only loosely. Balloons resist inflation because rubber stretches; lungs resist because of elastin fibres and the surface tension of fluid lining hundreds of millions of alveoli. Surfactant reduces that surface tension and stops small alveoli collapsing. A balloon has one chamber; a lung has an enormous branching tree.

What happens if I make a hole in the bottle?

The seal breaks, so changing the bottle’s volume no longer changes its internal pressure, and the balloon stops responding. That is mechanically analogous to a pneumothorax, where air gets into the space around a lung and the lung can no longer be inflated by chest movement.

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About the author

Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.

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Sources

1. De Troyer, A., & Boriek, A. M. (2011). “Mechanics of the Respiratory Muscles.” Comprehensive Physiology, 1273–1300. https://doi.org/10.1002/cphy.c100009
2. Ratnovsky, A., Elad, D., & Halpern, P. (2008). “Mechanics of respiratory muscles.” Respiratory Physiology & Neurobiology, 163, 82–89. https://doi.org/10.1016/j.resp.2008.04.019
3. National Heart, Lung, and Blood Institute. “How the Lungs Work.” National Institutes of Health. https://www.nhlbi.nih.gov/health/lungs
4. Harrison, A. G., & Treagust, D. F. (2000). “A typology of school science models.” International Journal of Science Education, 22, 1011–1026. https://doi.org/10.1080/095006900416884
5. Smith, J. P. III, diSessa, A. A., & Roschelle, J. (1994). “Misconceptions Reconceived: A Constructivist Analysis of Knowledge in Transition.” Journal of the Learning Sciences, 3, 115–163. https://doi.org/10.1207/s15327809jls0302_1
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165