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APPLIED FIZZICS · Field Notes · Edition No. 7
A whimsical look at the universe through a glass of Champagne...or your favorite cocktail.
August 11, 2026 · Seattle, Washington
Dear Reader,
Everyone knows what happens when you shake a can of beer. But do you know why? That was the question in last week’s quiz.
Last Week's Quiz
You vigorously shake a can of beer and hand it to your friend as a joke. The beer spews all over him when he opens it. Which statement is most accurate?
- A. Shaking releases more CO₂ from solution inside the can, increasing the pressure and causing it to spew when opened.
- B. The pressure in the can stays the same, but shaking creates countless tiny CO₂ bubbles throughout the liquid that act as nucleation sites when the can is opened.
- C. You're a real jerk.
We also asked a Bonus Question: Will tapping on the top of the can with your fingernail prevent spewing, and if so, why? This is a persistent urban legend, especially among college-age young adults, so we decided to test it in the video below.
The answer is B (and a little bit C, of course). And if that surprises you, you're in good company—because the obvious explanation is that shaking the can increases the pressure inside it.
So we put that explanation to the test. But first, let's see if tapping the can suppresses the spewing. Watch the first 45 seconds below for the definitive answer.
OK, now let's dig into what's going on inside the can—and find out what really causes the spewing.
To settle this, it would be great if we could attach an airtight pressure gauge to a beer can to see what happened to the pressure during shaking. And it would be great if we could see inside the can. But I couldn't think of a way to do this.
So instead, I decided I would transfer the beer to a clear plastic soda bottle, so I could see it during shaking.
Then, I built a simple device to measure the pressure inside that bottle.
The experimental procedure would be this: Transfer the beer to the bottle and affix the cap. Pressurize it with CO₂ to about 40 psi. Then put it in the refrigerator overnight to let the beer and headspace come to equilibrium.
Then the fun part: Shake the bottle and observe what happens on the gauge.
If shaking really causes more CO₂ to come out of solution, the pressure should go up.
So I shook it.
It didn't.
The beer turned almost completely to foam. But the pressure gauge barely moved.
What happened is this. After being in the refrigerator overnight, the CO₂ in solution in the beer was in equilibrium with the headspace. Shaking the bottle did not produce any significant net transfer of CO₂ into the headspace—the pressure barely changed.
But a lot of foam was created. The mechanical agitation of shaking can create foam even without carbonation—you can see the same thing by vigorously shaking tap water in a bottle with a drop of detergent at atmospheric pressure.
Those bubbles are the key. While the bottle remains sealed, the beer and headspace are still at essentially the same overall pressure. But when the cap comes off, those tiny bubbles suddenly find themselves in a completely different world.
First they expand as the pressure drops, and the existing foam suddenly exceeds the volume of the container. Then dissolved CO₂ begins pouring into them as the beer adjusts to its new equilibrium at atmospheric pressure.
Physics Corner
Physics Corner: One spew, two different processes
Watch the video carefully when the can opens. Two different things happen on two very different timescales.
First comes the explosion.
Before the can is opened, the tiny bubbles created by shaking are under several atmospheres of pressure. When the tab opens, the headspace pressure falls almost instantaneously toward atmospheric pressure.
The gas already inside those bubbles expands immediately.
For example, at 45 psi gauge pressure, the absolute pressure inside the can is about 60 psi, or roughly four atmospheres (one atmosphere is approximately 15 psi). When the can opens, that falls to one atmosphere.
In that first instant, before much additional CO₂ has had time to diffuse into the bubbles, the Ideal Gas Law, PV=nRT, tells us that dropping the pressure by a factor of four leads to the gas already in each bubble expanding by roughly a factor of four.
That's the violent initial spew you see the instant the can opens.
Then comes the foam.
The beer itself is still loaded with dissolved CO₂ appropriate to the much higher pressure that existed inside the sealed can. At atmospheric pressure, it is suddenly supersaturated.
Now CO₂ begins moving out of solution and into all those conveniently available bubbles. They continue growing, producing the mass of foam that rises out of the can and spills down its sides over the next several seconds.
So the spectacular mess actually has two acts:
First, compressed bubbles expand. That's the Ideal Gas Law. Then dissolved CO₂ feeds them. That's Henry's Law reasserting itself.
So there you have it. Shaking a beer doesn't turn up the pressure. It loads the beer with tiny bubbles just waiting for the pressure to come down.
And tapping the can?
Save your fingernail.
This Week's Quiz
At the end of the video above, I poured a flat beer into the bottle, put the pressure cap on, and pressurized the bottle with CO₂ to about 40 psi.
What will happen to the pressure gauge when I shake the beer?
- A. The pressure will go up.
- B. The pressure will go down.
- C. The pressure will stay essentially the same.
Why?
Make your prediction, then reveal the answer below.
Reveal the Answer
The correct answer is B.
The beer in the bottle is flat. So it is in equilbrium with the tiny pressure of CO₂ in the atmosphere BEFORE the bottle is pressurized.
After the bottle is pressurized, there is a greater concentration of CO₂ in the headspace than in solution. So CO₂ will diffuse from the headspace into solution over time, lowering the headspace pressure.
Shaking the bottle will greatly speed this process, by increasing the amount of liquid beer in contact with high-pressure CO₂ in the headspace. What might have taken hours only takes seconds with agitation. This is the essence of forced carbonation.
Any questions? Class dismissed.
Cheers,
Evan Wallace
President, Applied Fizzics Inc.
Makers of The Perlage System®
Want more Champagne science, carbonation experiments, and behind-the-scenes product stories? Visit the Applied Fizzics Field Notes archive .
