Will tapping a pop can keep the carbonation from exploding on opening? Plus: why do we tap cigarettes?

Dear Cecil:

Why do people, 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? I have always laughed at this. 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.

Cecil replies:

This is not a problem that requires infinite wisdom, Benj. This is a problem that requires enough neural organization to qualify as a vertebrate, 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 H2O under pressure. Shake up the can and you create thousands of micro-size bubbles. Each bubble offers a tiny surface where CO2 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? 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 Back Yard 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. 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 a madman or a genius — in either case somebody you want to listen to with 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.

Tap dancing, part one

Dear Cecil:

You missed the most obvious explanation for tapping on a soda can: tapping takes time, and with the passage of time the CO2 goes back into solution.

You raise a legitimate point, a sufficiently rare occurrence in these parts that I went right out to the Back Yard of Science to do another experiment. I got two cans of Coke Classic at room temp and shook them each vigorously 60 times. Setting both on the pavement, I opened one immediately and got a good-size gusher of froth. Then I waited 60 seconds and opened the other one. There was no gushing to speak of. Mind you, I had done no tapping.

So you may well be right that what tapping chiefly does is kill time, an issue to which I perhaps gave inadequate attention in my original column. But that doesn’t change my main point, which is that tapping per se doesn’t do squat.

Dear Cecil:

In your column about tapping Coke cans to reduce fizzing, you said people did this because of “the same suppressed macho ethic that makes people tap the ends of their cigarettes before lighting up.” Have you never smoked? As an occasional smoker, I learned as a teen the importance of “packing” one’s cigarettes before smoking. “Packing” is the process of compressing the tobacco toward the filter end, ensuring smooth burning. Unpacked cigarettes may burn unevenly, which can cause the cherry, or lit portion, to fall off. This process is usually accomplished by tapping the unopened pack repeatedly on the filter end, but an alternative method is to tap an individual cigarette before striking. (I use my watch face as the striking surface.)

I don’t believe this. Next you’ll be telling me you bury knots at crossroads to get rid of warts.

In a halfhearted attempt to be scientific I bought a pack of cigarettes. I tapped a couple and compared them with some untapped samples. The difference was slight.

Not being a smoker and having no desire to start for the sake of something this ridiculous, I didn’t light up to see what practical result my tapping may have had. But I did call up a couple of the major tobacco companies, and I also had this question posted to the Usenet. The result, if we eliminate certain extraneous data points, was unanimous agreement that I was right. The most anyone could offer by way of an excuse for this practice was that it may be a throwback to the days of hand-rolled cigarettes.

The best reply came from Bill Penrose of Aurora, Illinois:

The reason people tap cigarettes is because Humphrey Bogart did it. It is one of the rich vocabulary of gestures that cigarettes allow you to make. Tapping cigarette on end: Suave, sophisticated. Throwing cigarette on ground and grinding it out: I’m about to do something significant. Blowing smoke in someone’s face: (If a man) Let’s take it out in the parking lot. (If a woman) How fast can you get your clothes off? Holding smoke a long time and exhaling through nose: I’m thinking real hard. Blowing smoke out through ears: I have defective eustachian tubes. Going to sleep with cigarette in mouth: In the next scene the firemen will be putting out the fire. Lighting one cigarette from another: (If a war movie) You can share my foxhole anytime. (Between a woman and a man) We can share oxygen tents. Quitting a 3-pack-a-day habit overnight: My doctor just discovered a tumor the size of a cantaloupe.

My $0.02.

Tap dancing, part two

Master Cecil:

For years I have enjoyed your columns, and have always accepted what you write as “The Truth,” or as close to the truth as is generally possible. However, I have now lost faith in your ability to ferret out the straight dope.

Throughout the unfortunate years when I was a tobacco consumer, about 40 percent of the smokers that I encountered tapped either individual cigarettes or the entire pack before opening. Your sample population must be woefully biased against smokers. Also, it really does make a significant difference in the arrangement of the tobacco. There is sufficient momentum to further pack the tobacco: up to several mm is typical. And the result IS a more consistent burn of the coffin nail.

Please reaffirm my faith by some great stunt of mental prowess, like solving the problem of voter apathy in a democracy.

In despair,

Dear Cecil:

I bow to your superior intellect on most subjects. At risk of feeling the sting of your wit I must disagree with the pack/no pack conclusion. Perhaps you should let me conduct the experiment. I notice a great difference in the cigarettes I’ve used to test this. I can produce evidence supporting Paul’s earlier claim concerning his teen smoking experience. Dying of cancer sucks but at least my cherry will stay put.

Dear Cecil:

You are still missing the point. One taps a cigarette to compact the tobacco. This restricts airflow in the cigarette. The tobacco burns slower and thus (trust me) tastes better.

Cecil, Cecil, Cecil …

I smoke Marlboro “Regular King Size in a box” cigarettes, and although I was not around when the custom of packing one’s smokes sprang forth, I have, in the last 20 years of smoking, noticed a definite advantage (albeit dubious, as you’ll see) to doing so. I’m in the habit of packing the filter end of a new pack against the back of my weak hand using five sharp smacks, then rotating the pack on a horizontal axis 180 degrees and repeating. The tobacco mixture is drawn down into the cigarette roughly 3/8″. If I fail to do this, my smokes definitely don’t last as long (that’s the dubious benefit … I should be smoking less, not more). Cecil, even though the amount of tobacco is the same whether packed or not, the cigarette simply burns more slowly when packed more tightly.

Dear Cecil:

Am I the only survivor from the 1950s? In those days, no one wanted filter cigarettes. Cigarettes with no filter were routinely tapped on a thumbnail or watch crystal. Tapping produced two benefits: fewer shreds of tobacco would would come out in your mouth from the tapped end, and the other end had 2 or 3mm of empty paper tube for good ignition.

Dear Cecil:

I can’t believe no one could explain this to you! As a former smoker, I don’t recommend the habit, but I do have some experience to share. You are right, tapping makes little difference in whether the cigarette holds together or not. However, with a filtered cigarette, there is a difference in the “draw,” a difference I could notice when I smoked. I preferred the draw on a tapped cigarette. The practice probably started with unfiltered cigarettes, where it has a very practical purpose. With unfiltereds, you tap the end you light, packing the tobacco in the other end a good 1 to 2 millimeters from the end, and causing loose bits to fly out the end. This gives you a slightly recessed end to place in your mouth, allowing you to puff to your heart’s discontent without getting a mouth full of tobacco. Since I started with unfiltered Camels, I can attest to the difference between a tapped and untapped cigarette. It only takes a few episodes of spitting out little bits of tobacco, trying to dislodge it from under tongue and between teeth, to learn the practical purpose of tapping.

Sometimes there is no substitute for experience, Cecil.

Dear Cecil:

As a former smoker I can definitely say that tapping the “lighting” end of the cigarette loosens the tobacco and makes it easier to light in the wind.

Thanks for the letters, which represent only a fraction of the response we got. While I happily concede that you folks smoke (or smoked) and I don’t, I feel obliged to point that we now have at least six more or less independent theories on what cigarette tapping is supposed to accomplish:

  1. It keeps the cherry from falling off.
  2. The cigarette burns slower and thus tastes better.
  3. The cigarette burns slower and thus lasts longer.
  4. It kept tobacco bits out of your mouth in the 1950s (if you tapped the end you lit).
  5. It makes the cigarette easier to light by exposing more paper (if you tap the end you put in your mouth).
  6. It makes the cigarette easier to light by loosening the tobacco (if you tap the end you light).

In my experience this usually means:

  1. We don’t know why we’re doing this, but by God there must be some reason, and sooner or later we’re going to figure out what it is.

A few additional thoughts, using the term loosely

Dear Cecil:

Did you also know that having “tapped” or “packed” your cigarettes will help in keeping them lit in the rain?

Dear Cecil:

On the subject of tapping cigarettes. We all know they are bad, so we spank their little butts before smoking them.

Dear Cecil:

The Chesterfields-in-Casablanca legend needs a slight bit of filler-outer detail, to wit: by the time a pack of real Chesties got to Casablanca, they were so stale & full of weevils you would have had to pack it down to stun the weevils & prevent your getting a mouthful of fire, tobacco crumbs, & conscious weevils on your first hit.

Dear Cecil:

Even Jove nods.

I believe the reason your test didn’t produce any results is that being a non-smoker, you simply opened a fresh pack and tapped a few. In fact, freshly opened cigarettes are usually “fully packed.”

However, bouncing around in one’s pocket, especially after a few smokes are removed, gradually loosens the tobacco. As another correspondent has noted, this is especially true of non-filters, like the kind that killed Bogie. While the issue of loose strands in the mouth is not a problem with filters, tapping does produce a smoother burning, cherry retaining smoke.

Carry a half full pack around for a few days, then repeat the tap test. Not only will it enlighten you on this issue, but you’ll also experience the thrill of being a pariah when people see the pack in your pocket. For most dramatic results, I recommend Luckies or Camel straights.

Yours in undiminished admiration,

Yes, but for the full effect, shouldn’t I also get a tattoo?

Dear Cecil:

A packed cigarette can be “flicked better,” allowing you to keep the burning ember short without having to snub part of it off in the ashtray.

And, of course, my favorite observation:  A packed cigarette is easier to light for the kinda “trashed” girl in the bar that just bummed one from you.   This is because the newly created “little paper end” will catch fire before the tobacco. Invariably, in my experience, half-drunk people tend to lean forward and stick the business end of the cigarette into your match while breathing out of their nose if the cigarette doesn’t light instantaneously.  More often than not, either the butt snuffs the flame or the breath does.  If the little paper end flares up, they invariably draw on the cigarette and pull back slightly from it, thinking it’s lit.   Following the withdrawing butt with the match at this point for another half second ensures that it is evenly lit.

Dear Cecil:

I’m 33 and I’ve been smoking since I was 16. In all those years the only thing I’ve seen accomplished by “packing” cigarettes is the attention drawn to the teenager who is doing it. It seems like an action that tries to convey the message, “HAH! I’m a cool, smoking kid and I don’t care who knows it!” Stand outside any convienence store on a summer day and the sounds of “packing” are so loud, you’d think someone was putting a roof up somewhere.

Dear Cecil:

Those in the know understand the importance of clandestine signalling to indicate to fellow CSU (Cigarette Smoker’s Underground) members of the presence of evil non-smokers, legislative do-gooders and aliens from Zendarr who want the Sacred Weed for their journey home where it is easily converted to pzarkxs or purple wyrtlsdes. If you notice cigarette tapping but don’t understand it it may be time to report back to your commandant.

Long live the CSU!!


OK, in addition to the reasons summarized previously, we now have the following hypotheses on what cigarette tapping accomplishes:

  1. Easier to light cig bummed by tipsy bimbo.
  2. Flicks better.
  3. Easier to keep lit in rain.
  4. Stuns weevils, keeps away aliens, etc.

You know, I was never able to get the cigarette people to return my calls about this. Betcha I know why. They’ve been reading this godblessit thread.

Send questions to Cecil via cecil@straightdope.com.

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