Tapping the top of a soda or beer can before opening it, and tapping the end of a cigarette pack before opening it. People do it. It's annoying.
As I understand it, drinkers of canned refreshments think flicking the top of a the can before opening it will prevent a burst of liquid upon opening the vessel; and cigarette smokers who tap the carton are aiming at packing the tobacco tightly next to the filter so as to achieve a slower, smoother, more flavorful smoking experience.
Now, I don't really care to argue the efficacy of these tappings and whether they accomplish anything or not. Many have argued about the habits in the past and the general conclusion seems to be the practices may or may not achieve their ends. I am merely proposing that it is a habit as annoying a clipping your toenails on the subway or talking loudly on your cell phone in a crowded area, and should not be carried out in a public place.
I admit I have a long-held personal bias against these affectations (and I do consider them to be, at least in part, affectations). I first encountered can and pack flickers in college, and I may have developed my negative view of the actions because they were invariably carried out by jerks. Usually men. Jockish, boorish men who belonged to fraternities. They'd tap at their cans and they'd smack at their packs with such obvious, self-satisfied relish, you knew that they thought they were just the bomb for knowing such nifty tricks for making the brew not explode and their cig taste better. It was like a secret skeleton key they were sticking in the lock of a private club every time they hammered away at their oral pleasures. "Yeah, dude! I'm smakin' the pack. Smackin' the pack, dude. Makin' it happen."
I guess I still see the people who tap and flick in this light. They are show-offs, pompous jack-asses. I think they perform their little monkey tricks not out of fear of excessive carbonation or to achieve the perfect puff, but to cull a bit of attention to their innate coolness. Why? Because they always do it like a half-time show, in full view of the general public. Never sitting, always standing. Never do you see them quietly clicking at the soda can top in their cubicle, or giving the pack a few muffled claps while sitting on a park bench. No, it's always in the middle of the sidewalk or in the center of the party. They tap away, loud and clear, 10, 15, 20, a ridiculous amount of times until everyone sees them do it. Then they open their can or take out their perfect cancer stick as we supposedly revel in amazement and wonder how we could one day possibly be as in-the-know as they.
And, you know, there's another way to make sure your can won't erupt. Put it on a table and let it sit still for a couple minutes. Of course, nobody would notice that much, would they?