Why does blowing between two soda cans pull them together instead of apart?
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Why does blowing between two soda cans pull them together instead of apart?
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.
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.
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.