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human
ELI5: Why you can drink in public, so long as your booze is in a brown paper bag.

All right, I know this is probably self-explanatory, but hear me out.

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Six years ago, I was 16 years old and waiting in line for a Slayer show (Unholy Alliance Tour 2006, if you care enough to verify that I'm not talking out of my ass) and actually went out of my way to meet some really cool people outside of the show. 

One of them being 21 (and realizing that we were going to be outside for at least another two hours) proceeded to ask around if anyone wanted anything from the liquor store across the street. I was a little bit paranoid about this, considering that there were cops across the street just waiting for people to start crowding the place and throwing fits because they were denied admittance due to... I don't know... carrying a weapon, or something obvious like that. I voiced my concerns and the guy told me not to worry about it, so I gave him a few bucks and told him to get me the best bang he could with the money I gave him.

He comes back with two plastic bags (marked with the liquor store's name, mind you) packed to the brim with 40's and fifths, proceeds to pass them around, and gives me a 40 of Old English 800. I'm 16, I'm getting drunk, I love metal, and I'm seeing Slayer. I'm *stoked.*

A cop across the street starts to take notice to this nonsense and proceeds to get on her radio while opening her cruiser's door. The guy who bought us the booze passes everyone a paper bag who doesn't have one and says, "Wrap this around your drinks before cracking them open." I do so, but in my paranoid 16 year old mind, I'm still going to jail, so I tuck the bottle behind me and pretend it's not even mine.

The cop, God bless her, walks over with her hand resting on her belt looking like she was going to maim and detain every last one of us, takes one look around at the wrapped bottles, and just chuckles a little bit before asking us if we were keeping our noses clean, who we were there to see, and whether or not everyone was being civil in line thus far. 

We chatted her up for a good ten minutes while sipping our drinks (I warmed up and grabbed mine after a few minutes), she did everything but tip her hat, and then moved along back to the car. We proceeded to get proverbially shit-housed, tossed our empty bottles in the trash, went strolling into the venue feeling like vikings after a pillage, and enjoyed a show I could never hope to see bested. 

**tldr: Outside Slayer show; new friend buys my then 16 year old ass booze; cop takes notice; brown bags are passed around to cover bottles; I cringe; cop laughs; I laugh; I drink; cop goes away; we all see Slayer and go home to nurse our inevitable hangovers.**

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This cop obviously knew we were drinking booze. *Everyone* who's been on this planet longer than a few years knows that a brown paper bag in public equals alcohol, without a doubt.

Was she just being nice, or are there seriously laws in places that prevent police officers from hassling people for drinking in public, so long as the label is concealed?
gpt
She was just being nice. While they cannot legally search your bag without your consent, they can detain you until a warrant is filed or if there is probable cause to search (read:obvious intoxication).  Both of those take time.

The chatting may have just been a way to gauge how drunk you were.  She saw no over intoxication and didn't want to do paperwork.