Cabin crew are the unsung heroes of the sky - they bring us drinks, they make sure we're safe and, most of all, they have to deal with those difficult passengers.
Of course, they have to keep things professional, so sometimes use coded language when referring to certain customers or situations - whether it's alerting a colleague to an attractive passenger or subtle ways to communicate that there's a dead body travelling on board.
For the latter, one person who claimed to be a ramp agent for a US airline took to a Reddit thread asking people to reveal secrets passengers don't know.
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And it turns out if you ever hear 'HR' on the radio, just be aware that there might be an extra passenger on board.
"A lot of freight gets shipped on commercial flights," wrote user /mrmiguez.
"One of these items was always called HR on the radios. HR was an abbreviation for 'Human Remains'.
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"Some people die far away from where they want to get buried. They're packed in wood-framed boxes, so you would never know what was inside except by the strange shape of them."
But they're not the only ones lifting the lid on insider secrets.
A few years back, an anonymous cabin crew member also revealed what codes they used on UK plane staff forum cabincrew.com - mentioning an in-joke used to subtly tell others they think someone is good-looking.
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The rules for the little-known game are simple: every time a cabin crew member sees a passenger they think is attractive, they say 'Cheerio' while keeping a straight face.
The game only works, of course, when the passengers are leaving after the plane has landed, and the staff congregate at the door to say goodbye.
Yep, while it's usually the moment for an awkward 'Thanks!' as you disembark, still woozy from your mid-flight nap, it could end up being your lucky day.
"There's the cheerio game you can play when passengers disembark," the flight attendant explained.
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"When you're standing there going 'buh-bye, thank you, take care' etc when you see someone you fancy, you say 'cheerio.'"
"You need to do it with a buddy and the challenge is to keep a straight face."
So next time you're on a flight and one of the staff members says 'cheerio' on your way out, you may well be in there.
Topics: Travel, Plane Etiquette, Reddit