Originally Posted by vsevolod4

Generally you are only allowed to consume alcohol served by a flight attendant. Anything else, regardless of whether it is purchased duty-free or not, regardless of whether it was sealed or not, is technically verboten.

That said, it is against the rules at most sporting arenas to bring your own alcohol (ostensibly for issues of public safety, but in reality, so you can get overcharged $12 for a beer), but people have been known to sneak alcohol in hip-flasks or pouring vodka into 7-up bottles and the like, and drinking furtively so they don't get noticed.

So someone bent on doing the same onto a plane could either try to get alcohol past security (it used to be quite easy; now, you're pretty much restricted to the small minibar/airplane-sized containers in a zip-loc bag unless you want to sneak a non-metallic, discreet, hip-flask through the metal detector), or you would purchase at a Duty-free.

In the US and a few other places, duty-free merchandise is handed to you as you're boarding, offering fewer opportunities to do anything with it. In most countries, however, you have full access to the contents airside. In theory (whether sealed or not, typically not), you could go to a bathroom, open up the bottle, pour some of it into another more discreet container, and then surreptitiously drink out of it or pour it into the glass of Coke you get from the FA.

Or you could go to your local liquor store and buy a bunch of those little airplane-sized bottles; they'll cost you $2 or so a piece. You CAN take these through security in your Zip-loc bag, subject to the restrictions. They do have <3.3 ounces. But you still are not supposed to drink them on board. I guess if you drink Bailey's, you could order one Bailey's from the FA, pay the $5 (NB price is going up on many airlines), and then keep replacing the empty bottle with a full one that's in your pocket ...

Not that I'm advocating any of this, of course!

A sure-fire way in countries where you buy duty-free airside and you can take it right out of the store is to drink it before boarding ... you can carry on alcohol that you've already ingested :-)