While it may seem flight attendants are there to greet you as you board the plane and offer assistance with luggage, they're there for a much stricter purpose: To check you (and all the other passengers) out.

They're by the plane door, smiling and waving, while simultaneously making notes on all the passengers who step onto the aircraft. Whether it's your physical appearance or how you walk, you're automatically ingrained in the mind of the crew members. 

"Passengers think we are just greeting them at the door," explained Jay Robert to HuffPost. "But they’d be surprised at the number of threats we eliminate at that stage of the flight which would have caused a delay or even harmed their health and safety.”

They notice where you're sitting, if you have trouble lifting your own carry-on and more. Many crew members will make note "if you are an able-bodied passenger, in case we need help for any kind of emergency," explained Shreyas P, who has worked for "five" major airlines. 

"I’m looking for able-bodied persons who can assist with security problems inflight, as well as someone who appears willing and able to assist in an emergency evacuation. Typically, this is someone who is traveling alone and in street clothes, looks like they are in above average physical shape or is known emergency service personnel," Zac Ford echoed, according to HuffPost. 

Likewise, Janice Bridger, a 27-year explained, "If I see someone who is muscular, powerful, strong, physically fit, I memorize his/her face and make a mental note of where they are sitting," according to Southern Living. "I consider this person a resource for me.”

Flight attendants also notice if you're drunk, are kind to the passengers around you and if you avoid eye contact with them.

"[I notice] who makes eye contact with me and who doesn’t. More often than not, the ones who don’t make eye contact make me investigate... Are they scared of flying? Are they feeling okay? Are they dealing with a personal issue? These are things people don’t tell you outright, and a facet of my job is making sure everyone is having a comfortable flying experience,” explained Stephanie Mikel from Southwest Airlines.

While much of this information is provided in the paperwork, flight attendants also make note of what kind of books you're carrying. "If you are reading a medical journal... we know who to approach for First Aid," explained Shreyas.