What happens when you blow as hard as you can between two hanging soda cans?
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After you watchWhat happens when you blow as hard as you can between two hanging soda cans?
The short answer
Blowing between two cans pulls them together because fast-moving air has lower pressure (less sideways push) than the calm air around it. The still air on the outside pushes harder than the rushing air in the gap, so it squeezes the cans inward — and blowing harder pulls them in harder.
Try this next
- What if the cans hang farther apart? Widen the gap before you blow and predict: does the same breath still clap them shut, or does the stream spread out too much to win?
- What if you blow on the outside of just one can instead of between them? Aim your breath at one can's outer side and guess first — does it swing away or toward you? Watch which way the unbalanced push sends it.
- What if the cans were heavier? Imagine swapping the light cans for full ones and predict how hard you'd have to blow. Then check a real shower curtain — a heavy one barely creeps, a light one sticks fast.
Now you — bend it
- What if What if you blow at the SAME speed but slide the cans farther apart — does one breath still clap them shut?The push that wins is the gap between inside and outside pressure. As the cans separate, your stream spreads, slows, and mixes with calm air before it reaches each can — predict what that does to the weak inside push.
- What if What if you aim the stream past just ONE can's outer side instead of down the middle?Now the fast, low-pressure air is on that can's outside and calm full-pressure air is on its inside. Predict which way the unbalanced push sends it — and whether 'pull' is even the right word.
- What if What if you sent water down the gap at the same speed instead of air?Bernoulli works on any moving fluid, but water is about 800x denser than air, so the same speed drops the pressure far more. Predict whether the squeeze gets stronger or weaker — and why density, not just speed, sets the size of the push.
Can you prove it?Nothing 'sucks' the cans in — the calm outside air does all the pushing, and the inward force exists only while the air is actually moving. — Blow steadily until the cans clap shut, then stop the instant they meet: a real suction would hold them stuck, but they drift apart the moment the stream dies — so the inward force vanishes with the motion. Then run the asymmetry check: blow across the OUTSIDE of a single can and it swings the same way, toward the fast air, confirming the low-pressure side is always the side being pushed toward, never a pull from the stream.
Design your own test:Before you drag the slider up, predict the shape of the curve: does the inward swing grow steadily with speed, or ramp up faster than the speed itself? (The pressure drop scales with speed squared, so doubling the blow roughly quadruples the push — guess first, then watch the inside arrows shrink.)
Explain it to a 6-year-old: Air is always gently pushing on everything from all sides; when you blow fast through the middle, that fast air pushes softer, so the calm air outside wins and nudges the cans together.
The whole story
How it works
Air is always pressing on things from every side, and normally the push on a can's two sides is equal, so it stays put. When you blow a fast stream of air through the gap between the cans, that moving air has less sideways pressure to spare. Now each can has weak (rushing-air) pressure on the inside and strong (calm-air) pressure on the outside. The bigger outside push wins and shoves the cans together. The faster the stream, the lower the inside pressure, so the harder they clap shut.
What people get wrong
Many people assume fast-moving air pushes harder on whatever it touches, so blowing between two things should blast them apart. The opposite is true: faster air has lower sideways pressure, not higher. The cans are not being blasted apart or sucked in — they are being pushed together by the ordinary, higher-pressure calm air on the outside.
The catch
It feels like the fast air grabbed the cans, but there is no pull at all — the calm air on the outside does the pushing. And it only lasts while the air keeps rushing: the moment you stop blowing, both sides go calm, the pushes even out, and the cans drift back apart. The same low-pressure trick is useful elsewhere (it helps lift wings, sprays a perfume bottle, and pulls a shower curtain inward), but it always needs moving air to keep working.
Questions kids ask
Does the fast air suck the cans in?
No. There is no sucking or pulling. The fast air in the gap simply has lower pressure, so it pushes outward only weakly. The calm, higher-pressure air on the outside pushes harder and shoves the cans together.
Why does faster air have lower pressure?
Air has a fixed amount of energy. When some of that energy goes into moving fast, less is left for pressing sideways, so its pressure drops. This is Bernoulli's principle: in a steady flow, where the speed is higher the pressure is lower.
What happens when I stop blowing?
The cans drift back apart. The low pressure only exists while the air is rushing. Once you stop, both sides of each can have calm air again, the pushes balance out, and there is nothing squeezing them together.
Where else does this same trick show up?
Everywhere fast air or water makes a low-pressure zone: it helps lift airplane wings, sprays the mist from a perfume bottle, and pulls your shower curtain toward you when the water is running.
Talk about it
- Before we blow, guess: will the cans fly apart or clap together? Why do you think so?
- Where do you push the air to make them move — and where does the push that actually moves them come from?
- Can you think of a place at home where rushing air or water pulls something toward it?
For grown-ups
This is Bernoulli's principle: along a streamline in a steady flow, energy is conserved, so where the air moves faster its static pressure is lower. Blowing through the gap speeds up the air between the cans and lowers the pressure there; the higher ambient pressure outside then provides a net inward force. Nothing is suctioned — there is no pull, only an unbalanced push from the higher-pressure side. Note that real aircraft wing lift also depends on the wing deflecting airflow downward (Newton's third law), so it is more than Bernoulli alone.
Keep going
What else makes you wonder?
- Could you push two things apart with moving air instead of together?
- If still air is always pushing on you from every side, why don't you feel it?
- What other everyday things sneak toward each other when air or water rushes past?