To figure out how not to make your can runneth over, you first have to understand what makes soda fizz in the first place. Bubbles in soda are made of carbon dioxide (CO₂) gas, and cans are pressurized to keep that CO₂ fully dissolved in the liquid. Once you open the can, the pressure drops and allows the CO₂ to form back into gas, rise to the top of the liquid, and escape into the surrounding air.

It's actually pretty hard for CO₂ to form bubbles in an undisturbed can of soda. That's because the liquid's surface tension, combined with the pressure, makes it take a lot of energy for the gas molecules to separate the liquid molecules. Turbulence, i.e. shaking the can, creates bubbles on its own, however. Once a bubble is formed, it's a lot easier for that bubble to get bigger — that's because the larger the bubble's surface area, the less energy you need to break surface tension and increase the number of molecules inside.

Surface tension is also the reason diet soda fizzes so much more than regular soda. Diet Coke and Pepsi, Coke Zero, and many other sugar-free sodas are sweetened with aspartame. Like soap in water, aspartame acts as a "surfactant," lowering the surface tension of the soda much more than sugar or corn syrup does. With less surface tension, gas dissolved in diet soda needs less energy to form bubbles, resulting in way more foam than its full-sugar brethren.

Now that you know why sodas fizz, how do you keep it from happening? The sure-fire method is just to wait, since the calmer the soda is, the fewer bubbles will have already formed (since remember, it's easier to make an already formed bubble get bigger than it is to form a bubble in the first place). You can also chill the soda since lower temperature means lower pressure, which means less of a pressure difference when you finally crack open the can.

But yes, tapping the can can work. When bubbles form via turbulence, they tend to stick to the sides of the can. The idea is that if you can free those bubbles, they'll rise to the top, forming one big bubble that harmlessly escapes as gas when you open the can instead of taking a bunch of liquid along for the ride.

The problem is that when most people tap a soda can, they focus on the top. That neglects the bubbles clinging to the walls and bottom of the can, leaving them free to rise violently when the pressure released. But if you tap on the sides of the can, you may be able to free those bubbles, helping them rise to the top and escape harmlessly. We experimented with the two different tapping methods ourselves in the Curiosity office and were pleasantly surprised by how effective tapping the sides can be. Give it a try on your next can of soda!

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