Pneumatics Experiment for Kids: Lift Weight With Air

An eight-year-old can lift ten kilograms of books with one breath. Not with a lever, not with a pulley: by blowing into a drinking straw taped into the corner of a freezer bag with the books resting on top. Force equals pressure times area, so a gentle few kilopascals spread over a 20 by 15 cm patch adds up to well over a hundred newtons. And once the books are up, this pneumatics experiment for kids turns into something better: fill the same syringe rig with air and then with water, and you can measure the one property that separates pneumatics from hydraulics completely.

Key Takeaways

• You’ll lift a stack of books with one breath through a freezer bag, then compare an air-filled syringe rig with a water-filled one.
• The concept is that air compresses and liquid barely does. Air stores energy like a spring; water transmits motion exactly.
• The number that explains everything: air at normal pressure has a bulk modulus of about 0.1 megapascals, while water’s is about 2.2 gigapascals, roughly 20,000 times stiffer.
• You can also calculate your own breath pressure from the mass you lifted and the contact area.
• Best for ages 8 to 14. The only real cautions are no needles on the syringes and no plastic bags near small children’s faces.

What You’ll Need for the Pneumatics Experiment for Kids

Item	How much	Where to get it	If you don’t have it
Zip-top freezer bags, 3–4 litre size	3	Supermarket	A sturdy sandwich bag, which lifts less
Drinking straws	4	Kitchen	20 cm of aquarium tubing
Oral or dosing syringes, no needles, 20 mL	2	Pharmacy	Two 10 mL syringes, with smaller effects
Aquarium or silicone tubing to fit the syringes	50 cm	Pet shop, hardware shop	A drinking straw sealed to the syringe with tape, which leaks more
Masking tape	One roll	Hardware drawer	Electrical tape, which seals better on plastic
Stiff board or chopping board	1	Kitchen	A hardback book acts as both board and weight
Books or 1 kg rice bags	Up to 15 kg	Shelf, kitchen	Bottled water, 1 kg per litre
Kitchen scale	1	Kitchen	Use labelled 1 kg bags instead
Ruler	1	Stationery	A printed paper ruler
Jug of water	1	Tap	Any container

Time: prep 20 min, experiment 35 min, cleanup 10 min Cost: a packet of bags and two pharmacy syringes Mess level: low for the air part, medium once water is involved

Do the Experiment

Step 1 — Build the bag lift

Cut a 1 cm slit in one bottom corner of a freezer bag, push a straw 3 cm inside, and tape it round and round so no air escapes. Seal the bag’s zip. Blow gently and check for leaks by listening and feeling; a leaky seal is the reason most first attempts fail.

Step 2 — Measure the contact patch

Lay the bag flat on a table, put the board on top, and press down. Mark the outline of the bag where it touches the board and measure it. A typical flat freezer bag gives roughly 20 by 15 cm, which is 0.03 square metres. Write the number down, because it turns the lift into a measurement.

Step 3 — Lift as much as you can with one breath

Stack books on the board, starting with 2 kg. Blow a long steady breath through the straw. Add 1 kg at a time and repeat until the lift fails. Record the heaviest mass that rose off the table even slightly.

Step 4 — Calculate your breath pressure

Pressure equals force divided by area. Take the mass in kilograms, multiply by 9.81 to get newtons, divide by your contact area in square metres. If 8 kg lifted over 0.03 m², that is 78.5 newtons over 0.03 m², which is about 2.6 kilopascals. That number is your child’s breath pressure, and it is a real measurement with a real unit.

Step 5 — Build the syringe pair and run it on air

Pull one plunger fully out to 20 mL and push the other fully in, then join them with the tubing, sealing both joints with tape. Hold the output syringe’s body and push the input plunger in by exactly 10 mL. Measure how far the output plunger moved. Then push and release repeatedly and feel the springiness.

Step 6 — Refill with water and compare

Separate the syringes, fill both and the tubing with water with no bubbles left, and reconnect. Push the input plunger in by 10 mL again and measure the output. Push and release and feel the difference. Record both input-output pairs in a table.

What you should see: The bag lift usually surprises everyone. Most children manage 6 to 12 kg, and the board rises smoothly and slowly rather than popping up. The air-filled syringe pair feels bouncy: pushing 10 mL in moves the output by perhaps 5 to 8 mL, the rest disappearing into compressed air and stretched tubing, and letting go springs the plunger partway back. The water-filled rig is a different machine entirely: 10 mL in gives close to 10 mL out, it moves the instant you push, and it does not spring back. One bubble left in the water ruins the effect, which is itself worth noticing.

What the Pneumatics Experiment for Kids Measures

Measurement	Air	Water
Output movement for 10 mL input	5–8 mL	9–10 mL
Response when you push	Lag, then movement	Immediate
Response when you release	Springs partway back	Stays put
Bulk modulus (resistance to compression)	~0.1 MPa at 1 atm	~2.2 GPa
Good for	Gentle grip, cushioning, bouncing	Precision, stiffness, heavy lifting

What’s Actually Going On

Two ideas carry this whole experiment, and they are worth keeping apart.

The first is that pressure acts on area. A gas pushes equally on every surface it touches, so the upward force on your board is the pressure inside the bag multiplied by the area of contact. Breath pressure is modest, a few kilopascals, but a kilopascal is a thousand newtons per square metre, so spreading it over 0.03 m² gives you tens of kilograms of lifting force. This is exactly how a car lift, a pneumatic jack and an air-powered riveter work: unimpressive pressure, generous area. The bouncy castle at a birthday party is the clearest everyday example, held up by a continuously running blower keeping the inside only slightly above atmospheric pressure, spread across an enormous floor.

The second idea is compressibility, and this is the one that separates air from liquid. Squeeze a gas and it genuinely gets smaller: Boyle’s law says pressure times volume stays constant at a fixed temperature, so halving the volume of trapped air doubles its pressure, as set out in NASA Glenn’s page on Boyle’s law. Squeeze water and essentially nothing happens. The quantity that measures this is the bulk modulus, the pressure change needed to produce a given fractional change in volume. For air at atmospheric pressure the isothermal bulk modulus equals the pressure itself, about 0.1 megapascals. For water it is roughly 2.2 gigapascals, a figure you can find in HyperPhysics’ elasticity tables, with more precise values across temperature and pressure in NIST’s thermophysical property database. That is a ratio of around twenty thousand.

Twenty thousand is why your two syringe rigs feel like different machines. The blog already has three articles on hydraulics, including hydraulics with syringes and tubing, and the thing to draw out when a child has done both is the contrast rather than the similarity. A hydraulic system moves the output the instant you move the input, because the liquid has nowhere to go and cannot shrink. A pneumatic system swallows part of your input into compressing the air, then releases it later. That lag is a problem if you want precision, and it is a gift if you want gentleness.

Engineers choose between them on exactly that basis. Excavators and car brakes are hydraulic because they need stiff, precise, high-force control. Factory grippers, dentists’ drills and bus doors are pneumatic because air is clean, leaks harmlessly and cushions what it touches. Soft robotics has made the trade-off central: Mosadegh and colleagues’ 2014 paper in Advanced Functional Materials describes soft robots actuated by inflating networks of small channels in rubbery materials, which are “appealing for producing sophisticated motions with simple controls”, and notes that earlier designs of these pneumatic networks actuated relatively slowly, over seconds. Rus and Tolley’s 2015 review in Nature sets out the wider field of robots built from compliant materials, where deformation is the point rather than a defect.

One correction worth making plainly. People often say air is “weaker” than hydraulic fluid. It isn’t; pressure is pressure, and compressed air at 700 kilopascals pushes just as hard as oil at 700 kilopascals. What air is, is springy. The difference is not strength but stiffness, and conflating the two is the most common misunderstanding of the whole subject.

How to Explain It to Your Kid

Ages 5–8: squash the air, squash the water

Let them lift the books with their breath first, because that is the moment worth having. Then give them the air syringe rig and let them push and feel it bounce back. Then the water one, which doesn’t. Air can be squished, water can’t. That is an entirely sufficient version at this age, and they will have the physical memory for years.

Ages 9–12: force equals pressure times area

Do the breath pressure calculation together. Mass, times gravity, divided by area. Then ask the extension question: if we used two bags side by side, doubling the area, what mass could you lift with the same breath? Then test it. Doubling the area should roughly double the lift, and confirming that with books is far more convincing than being told.

Ages 13+: Boyle’s law and bulk modulus

Have them block the output syringe entirely and push the input in from 20 mL to 10 mL, then use Boyle’s law to work out the pressure inside, about 202 kilopascals absolute, or one atmosphere of gauge pressure. Then introduce bulk modulus and the twenty-thousand-fold ratio. Finally ask them to explain, using those numbers, why a car’s brake lines must be bled of air bubbles. The answer is the whole point of the article.

The question to ask: “If you wanted a robot hand to pick up a ripe strawberry without crushing it, would you make it hydraulic or pneumatic, and what exactly makes that the right choice?”

Why Teaching This Early Pays Off

The distinction between force and stiffness runs through engineering and almost never gets taught explicitly before university. A pneumatic cylinder and a hydraulic cylinder at the same pressure deliver the same force; only one of them holds its position under a changing load. A child who has felt a syringe spring back has met that distinction physically, and the words for it will land easily later.

It also pre-loads gas laws in a way that chemistry lessons rarely manage. Boyle’s law is usually a formula about an abstract container. Here it is a plunger that pushes back harder the further you compress it, which is the same thing but impossible to forget.

Keep the explaining with the child. The National Research Council’s framework places constructing explanations among the core practices, and this experiment produces two results that beg to be explained: the surprisingly large lift, and the springiness. What it does produce is a child who notices the hiss of a bus door and knows why it is air rather than oil. If you want the atmospheric-pressure companion to this, there’s crushing a can with air, and for more air-powered builds the pneumatics and air pressure projects go further.

Before You Start: What Parents Should Know

Best age	8–14. Under 8, do the bag lift and the feel comparison, and skip the calculations
Supervision	Light, with an adult present for the syringe work and for all bag handling around younger siblings
Real hazards	Plastic bags are a suffocation hazard: never put a bag near anyone’s face, keep every bag away from children under three, and count them into the bin afterwards. Use oral or dosing syringes with no needle, from a pharmacy; never a needle syringe. Don’t aim a loaded syringe at anyone’s face. One straw per person for hygiene. A bag that bursts is loud and startling rather than dangerous, but it will spray any water inside it
Don’t	Never pressurise a rigid sealed container, a glass jar or a bottle with a pump. Don’t use canned compressed air, which is cold enough to cause skin injury and is not a toy. Don’t connect a bicycle pump to the freezer bag; your lungs are the correct pressure source here

When It Doesn’t Work

• The bag deflates as fast as you blow. The straw joint leaks. Wrap tape in a spiral along the straw and onto the bag, press the plastic firmly round the straw, and test by blowing with the bag’s far end pinched shut.
• The books don’t move at all. Too much mass for the area, or the bag is folded under itself so its contact patch is small. Start at 2 kg, flatten the bag completely, and make sure the board sits squarely over the bag rather than off to one side.
• The air syringe barely moves the output. Tubing is too stretchy or a joint is leaking. Silicone aquarium tubing works well; soft, thick-walled tubing swells and absorbs your input. Tape every joint and re-test.
• The water rig has bubbles and behaves like the air one. Even one bubble restores springiness. Fill both syringes and the tube under water in a bowl, connect them submerged, then lift the whole thing out.
• The output plunger shoots out. You pushed too hard with the output unrestrained. Hold the output syringe body, keep it pointing away from faces, and push the input plunger slowly.

Take It Further

1. Area sweep. Repeat the lift with one bag, then two side by side, then three, keeping the same board. Plot lifted mass against total contact area. The line should be straight and its slope is your breath pressure, which turns one measurement into a graph.
2. Compressibility measured. Block the output syringe, then record the input plunger position against how hard it is to push, estimated with weights stacked on the plunger. The relationship curves, because pressure rises as volume falls. Water produces almost no travel at all, which is the same experiment giving a flat line.
3. Build a pneumatic gripper. Two cardboard jaws hinged with tape, opened by a syringe-driven push rod. Then try to pick up a cherry tomato, first with the air rig and then with the water rig. The air version is gentler, and the reason is the whole article. For the hydraulic counterpart, there’s the hydraulic excavator arm.

What to Watch For Over the Next 3 Months

• Week 4: Noticing pneumatics in the wild. The hiss of a bus door, an air compressor at a garage, the gentle close of a shop door, a bouncy castle’s blower running continuously.
• Month 2: Using area in an argument. Statements like “it’s not much pressure but it’s a big area” applied to something you didn’t set up.
• Month 3: Distinguishing force from stiffness. If your child says something is strong but springy, or firm but weak, and means it precisely, this idea has landed for good.

Frequently Asked Questions

How can a child’s breath lift ten kilograms?

Because force is pressure multiplied by area. A breath delivers only a few kilopascals, but a kilopascal is a thousand newtons per square metre, so spreading it across a flat bag of about 0.03 square metres gives well over a hundred newtons.

Why does the air syringe feel bouncy and the water one stiff?

Because air compresses and water essentially doesn’t. Air at atmospheric pressure has a bulk modulus of about 0.1 megapascals, while water’s is around 2.2 gigapascals, so air is roughly twenty thousand times easier to squeeze. Part of your push goes into shrinking the air rather than moving the far plunger.

Is a pneumatic system weaker than a hydraulic one?

No, and this is the most common misunderstanding. At the same pressure, air pushes exactly as hard as oil. What differs is stiffness: a pneumatic actuator yields and springs when the load changes, while a hydraulic one holds position. Choose hydraulics for precision and force control, pneumatics for speed, cleanliness and gentleness.

Can we use balloons instead of freezer bags?

You can, and it works, but the measurement gets worse. A balloon’s rubber stores energy and its contact area changes constantly as it inflates, so your breath-pressure calculation stops being meaningful. A flat zip-top bag keeps the contact area roughly constant, which is what makes the arithmetic honest.

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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. NASA Glenn Research Center. Boyle’s Law. Beginner’s Guide to Aeronautics. https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/boyles-law/
2. Nave, R. Elasticity, Elastic Properties. HyperPhysics, Georgia State University. http://hyperphysics.phy-astr.gsu.edu/hbase/permot3.html
3. National Institute of Standards and Technology. Thermophysical Properties of Fluid Systems. NIST Chemistry WebBook. https://webbook.nist.gov/chemistry/fluid/
4. Mosadegh, B., Polygerinos, P., Keplinger, C., Wennstedt, S., Shepherd, R., Gupta, U., et al. (2014). “Pneumatic Networks for Soft Robotics that Actuate Rapidly.” Advanced Functional Materials, 24(15), 2163–2170. https://doi.org/10.1002/adfm.201303288
5. Rus, D., & Tolley, M. T. (2015). “Design, fabrication and control of soft robots.” Nature, 521, 467–475. https://doi.org/10.1038/nature14543
6. National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. https://doi.org/10.17226/13165