An experiment that anyone can do is to go into an English supermarket, buy some stuff, and after the cashier is finished beeping your stuff through, ask for a packet.
What will happen is that they will look at you with complete, complete incomprehension. Sometimes they will look angry also, or else a bit frightened, but the thing they will look the most is entirely confused.
"Can I have a packet please?"
"What?"

Like that.
It drove me demented. Mae also. Then you would say, just louder, "Could I have a packet please?" and they would say "I'm sorry, but what did you say?" like you had given them some bad news about their family. And then you mime what a packet looks like (you sort of sketch a circle in the air with a half circle lying on top), and then they say, always, "ohhhhhhhh, you mean a BAG!"
Like all pleased with themselves.
It really does happen every time. Once I asked Dan What The Fuck, and he said "it's because a packet is something different here."
I said, "How different can it possibly be?"
and he said "oooooh, it's just different", and sketched more of a square in the air.
He said "You put your money in a packet"
I said "No you don't you put your money in a wallet I have seen it over and over."
He said "Look, it's just one of those things"

If by one of those things he means "the English tendency to look theatrically bewildered when you don't understand something, in a way that makes the other person feel like they have been actually rude", then yes okay.

Towards the end, I would remember sort of halfway through and say "Can I have a pag, please?"
And they would say "a WHAT?"
and I would just start screaming my head off in the middle of Waitrose.