I have always laughed at people who, before opening a carbonated drink which has been shaken, tap the top of the can with their finger so that it doesn’t explode upon opening. After lengthy arguments, we even performed a semiscientific experiment by shaking a drink and opening it with and without tapping the top, but with no solid scientific conclusion. We would like to know what you, in your infinite wisdom, think of this. –Benjie Balser, Dallas, Texas

This is not a problem that requires infinite wisdom, Benj. This is a problem that requires an IQ above two, apparently a stretch for some folks these days.

First I called the folks at Coke central in Atlanta. I did this in the interest of thoroughness, in case Coke physicists had discovered quantum mechanical aspects of beverage carbonation that had previously eluded the notice of science. However, they didn’t return my calls. There are two possible explanations for this: (1) everybody was out in the plant stamping out souvenir Olympic bottles, or (2) Cecil’s message was a little too detailed. This is an inherent risk in my business. If you tell some low-level gatekeeper type you have a question about poultry, you may actually get through. Tell them you want to know which end of the egg comes out of the chicken first, and they’ll have security trace the call.

No matter. First let’s consider the matter from a theoretical perspective. Carbonation is produced by forcing carbon dioxide into solution with H<2>O under pressure. Shake up the can and you create thousands of micro-size bubbles. Each bubble offers a tiny surface where CO<2> can rapidly come out of solution, creating the potential for explosive fizzing should you open the can prematurely. Wait a while though, and the bubbles will float to the top of the can and disappear, and eventually all will be as before.

But suppose you’re the impatient type. You tap the can. What, pray tell, is this supposed to accomplish? Are we going to noodge the tiny bubbles to the surface faster, after the manner of herding cows? Right. Are we going to maybe dislodge a few bubbles that have stuck to the sides of the can? Maybe we are, but the difference is slight. Open that baby and you’re still going to get a faceful of froth.

We confirmed this to our satisfaction out in the Straight Dope Backyard of Science with a half dozen cans of pop. OK, so I didn’t replicate my results 50,000 times. I figure if extraordinary claims demand extraordinary evidence, stupid claims demand well, something a little less rigorous.

I should tell you that when I had Little Ed broach this issue recently on the Usenet he heard from a science teacher, among others, who insisted tapping the can really did reduce fizzing and bragged about a classroom demonstration he did to make just this point. No wonder today’s youth are going to the dogs, I thought. But to be sure, I called up physicist Jearl Walker, who’s written about the physics of beverage carbonation in Scientific American. Jearl, you’ll remember, is the guy who used to plunge his hand into a vat of molten lead as a classroom demonstration of the Leidenfrost effect. This makes him either certifiably crazy or a genius–in either case somebody you want to listen to with an attitude of respect.

Jearl had heard similar claims about the efficacy of tapping and had a similar reaction: these guys are nuts. He said he could only attribute the persistence of the practice to the same suppressed macho ethic that makes people tap the ends of their cigarettes before lighting up.

If you want a real solution, try this. It’s an implacable fact that a warm can of pop that’s all shook up will fizz more than a shaken cold can. If you absolutely must pop the top on that jug of Jolt, stick it in the fridge first. You’ll chill the contents and chill the carbonation too, an inevitable consequence of increased gas solubility and Charles’s law.


Is there something you need to get straight? Cecil Adams can deliver the Straight Dope on any topic. Write Cecil Adams at the Chicago Reader, 11 E. Illinois, Chicago 60611, or E-mail him at cecil@chireader.com.

Art accompanying story in printed newspaper (not available in this archive): illustration/Slug Signorino.