If you travel frequently, you probably know airplane rules like the back of your hand. You can’t take liquids over a certain size onto the airplane, you need to put your tray table into an upright position before you land, you can’t go running down the aisle screaming while tearing off your shirt—for the most part, it’s pretty simple stuff.

However, most of us follow those rules without ever thinking about them. How did the Transportation Security Administration (TSA) decide that 3.4-ounce travel bottles were safe, while 4-ounce travel bottles were absolutely unacceptable? Why do we have to lift the window shades during takeoff and landing?

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To get answers, we spoke with a flight attendant and dove deep into airline regulations (which isn’t nearly as exciting as it sounds). Here’s what to know before your next trip.

“Please listen carefully to the pre-flight safety demonstration.”

Yes, the flight attendant’s safety speech is required by regulation, even if you’re flying on a red-eye with two other seasoned passengers.

“Passengers on short or [low-attendance] flights sometimes ask if we can skip the pre-flight safety demonstration,” says Jenny (not her real name), a St. Louis–based flight attendant. “No, we can’t. Even if we feel like skipping it, we have to do it.”

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It’s worth noting that flight attendants don’t necessarily need to be the ones performing the demonstration. Federal regulations simply require that every flight includes an “oral brief” for passengers; some flights have used pre-recorded videos, but live demonstrations are much more common.

The demonstration must explain how to use safety belts and oxygen masks, along with the location of emergency flotation devices, emergency exits, and fire extinguishers. Most airlines will have one crew member give the oral presentation while another crew member acts out putting on the seatbelt and oxygen mask. That’s not just because the second crew member feels left out.

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“People ask why we act it out,” Jenny notes. “That’s simple if you think about it: Many of our passengers don’t speak English.”

Some flight crews use humor to try to make the presentation more engaging for passengers (if you’ve taken a Southwest flight recently, you know what we’re talking about). That might seem like a harmless way to make flights less stressful—and it is—but it’s also a great way to ensure that passengers remember the important bits.

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A 2013 study tested three different pre-flight safety videos: One used humor, one didn’t use humor, and one featured a celebrity. Researchers found that including humor or a celebrity in a pre-flight demonstration “proved effective in terms of memory for key safety messages, and the video containing humor was the only video to positively influence individuals mood.”

Keep that in mind the next time you’re rolling your eyes at a crew member’s lame jokes; while you might not care for a flight attendant’s cheesy Darth Vader impersonation, it’s actually improving the quality and safety of the flight.

“Please limit liquid containers to 3.4 ounces.”

On the surface, this might seem like one of those pointless TSA rules invented for an imaginary threat. Are liquid containers really more dangerous when they hold more than 3.4 ounces of something?

Well, yes, according to a 2008 blog published on the TSA’s website. Kip Hawley was the TSA administrator at the time, and he directly addressed some of the common questions he received from passengers flummoxed by the “3.4 ounces” rule.

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“We are involved in risk management. The question to me is: ‘What do you have to do to make a successful attack so complex that an intelligent enemy would recognize that the odds of success are too low?’” Hawley wrote about the regulation.

He noted that he couldn’t reveal some of the classified information that led to his decision, but that experts had “demonstrated to my satisfaction that there is, in fact, a scientific basis for allowing small amounts of liquids on as carry-on.”

That’s why the TSA didn’t ban liquids from flights outright.

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“We try to prohibit the minimum possible from a security standpoint,” he explained. “Also, the consequence of banning all liquids is a large increase in the number of checked bags, which creates its own issues.”

Another common criticism of the rule: If someone really wanted to bring liquid explosives onto a plane, why couldn’t they simply bring multiple 3.4-ounce containers and mix them during the flight? The short answer: It’s complicated, but they couldn’t do that.

The longer version: It would take much more than a few 3.4-ounce containers to create a real threat, and by requiring travelers to put their liquids into containers—which then need to be placed in plastic bags—the regulations impose enough safeguards to keep flights safe.

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“Whatever you think about our policies, please recognize our Security Officers who train and test every day and will do whatever it takes to make you and your families safe when you fly,” Hawley wrote. “They are the best in the world and are on your side; please give them a little recognition when you see them.”

So how did the TSA decide that 3.4 ounces was the perfect amount to allow? It seems like an arbitrary number—until you realize that 3.4 ounces is exactly 100 milliliters. Initially, the TSA told American travelers that the allowable amount was 3 ounces, but that changed when the European Union joined the 3-1-1 program.

“In order to align with the EU, we decided to allow liquids in containers up to 3.4 [ounces],” a TSA blogger explained in 2009. “We also decided to keep our signage the same to maintain consistency.”

“From a marketing perspective, 3 ounces was easier to remember than 3.4. For the European Union, 100 milliliters was easier to remember than 89. So behind the scenes, we’ve been allowing up to 3.4 ounces, but it hasn’t been reflected on the web or in signage.”

“Put your tray up before the landing.”

The simple reason for this rule is that Federal Aviation Administration (FAA) regulations require it; we’re probably not telling you anything that you couldn’t have guessed.

“Some people become remarkably obstinate when you ask them to put their tray tables up or their seats in an upright position,” Jenny says. “I get that you probably feel like nothing bad will happen if you ignore us, but it’s a regulation. Please make things easy for us and just follow our instructions.”

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Even if you’re not particularly worried about annoying your flight attendant—in which case, shame on you—there’s another good reason to sit up and lock your tray in place.

“[Tray tables] can block you from moving around in an emergency,” Jenny says. “Most accidents occur during takeoff and landing, so that’s when it’s really important to have the ability to move through the aisles and get to an emergency exit.”

A 2014 study from Boeing (link opens a PDF) found that 60 percent of fatal airplane accidents occur during takeoff or landing. That makes sense; as the old pilot’s adage goes, anyone can fly a plane—landing is the hard part.

By the way, that also explains why flight attendants will ask you to put your phone, computer, and other items away before the plane touches down on the runway. If something goes wrong, loose items become projectiles, and nobody wants to end up with an iPhone-shaped indentation in their forehead.

“Not everyone can sit in the emergency seat.”

We love sitting in the emergency exit row. You get extra legroom without paying much more (that depends on the airline, of course).

But those seats come with additional responsibilities, and if you’re not capable of taking on those responsibilities, sit somewhere else.

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FAA regulations require passengers sitting with direct access to an exit to have “sufficient mobility, strength, [and] dexterity in both arms and hands, and both legs” to open the door an emergency. Basically, while the chances of a disaster are slim, if you need to make an emergency landing, you’re going to have to open the door. Your flight attendant will give you instructions before your plane takes off.

“Please don’t take [the emergency seats] for granted,” Jenny says. “And please don’t argue with us if we tell you that you have to sit somewhere else.”

If you have a disability, you’re under 15 years of age, or you’re unable to impart information to your fellow passengers easily—for instance, if you’re on an international flight and you don’t speak the same language as the majority of your fellow passengers—you’ll need to move on.

Some people simply don’t want the responsibility. If you feel like you’re not up to the task, ask your flight crew to relocate you. They’ll be happy to help you find another seat.

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“While giving the instructions [for the emergency seat], we’ve seen people look like they’re getting sick,” Jenny says. “They really don’t want to think about an emergency. Maybe they’re already a little bit worried about flying, and now they’re going to think about opening that door for the rest of the flight. If that’s you, don’t worry, we don’t think any less of you. Just—please, speak up, and we’ll put you somewhere else.”

“Shades up during take-off and landing.”

This one seems especially arbitrary. What if you don’t relish the thought of watching the ground recede below you as you ride into the sky on a giant hunk of metal? Flight attendants aren’t being needlessly cruel when they ask you to open the shades, though. Visibility is directly tied to safety.

“If, God forbid, we have to evacuate, I want to move fast,” Jenny says. “I need to see what’s out there to pick an evac route, and I can’t spare any time opening windows.”

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The Federal Aviation Administration requires flight crews to be able to complete a “full-scale emergency evacuation” within just 90 seconds. That means everyone on the plane, including the crew, must be able to disembark in just a minute and a half. It’s not easy.

It’s a little thing, but it could make a big difference in an emergency.

Besides, with windows open, flyers are more likely to spot problems with the plane, said Aviation Safety Officer Saran Udayakumar in the Daily Express.

“Passengers are curious; hence they are the perfect extra eyes to see if something goes wrong out there,” Udayakumar said. “Usually passengers report stuff right away.”

So why enforce this rule only during takeoff and landing? Those are the most dangerous portions of the flight, reported USA Today. Being so close to the ground doesn’t give pilots a chance to right the plane if something goes wrong. Besides, the slower speed of the craft during these portions of the flight makes it harder to control.

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Almost half of accidents on commercial jets occur during final approach and landing, according to the Flight Safety Foundation. Around 14 percent of accidents take place during takeoff and initial ascent. It makes sense to be extra cautious during these stretches of flight.

“We don’t feel like it’s that big of an ask,” Jenny says. “It’s a little thing, but it could make a big difference in an emergency.”

“Put on your own oxygen mask before helping your child.”

In an emergency, maternal instinct tends to kick in. When the oxygen masks drop, many parents want to take care of their kids first. To do otherwise feels selfish. The truth is that it’s anything but. In fact, the parent-first oxygen plan could end up saving the child’s life.

If something goes wrong and the cabin loses pressure, there won’t be enough oxygen in the ambient air to keep your body functioning normally. Lack of oxygen in the body is called hypoxia, and it can quickly impair your ability to complete even simple tasks. Hypoxia can even knock you out entirely.

According to the aviation safety site SKYbrary, you might only be capable of acting for seconds following depressurization. At an altitude of 35,000 feet, the site explains, the time of useful consciousness (yes, that’s a real term) in a depressurized cabin can be as little as 15 seconds.

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If you spend that 15 seconds struggling with your kid’s mask, you might pass out before being able to put on your own or even finish with your child’s. That’s a potentially fatal situation.

So that’s why flight instructions tell parents to put their oxygen mask on before attending to their children. If they don’t, they might be so impaired by hypoxia that they won’t be able to help anyone at all.

“Phones and devices off or in airplane mode.”

Aren’t flights boring enough without having to go without social media for hours at a time? Well, apparently there’s a pretty good reason flight attendants tell us to cut ties with our cell service while we’re in the sky.

“The idea is that radio pollution from your devices could mess with the flight, but I’ve never seen it happen. I don’t think anyone really cares if you leave your phone on,” Jenny says. “I forget to turn mine off all the time. They’re just being extra careful.”

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The truth is, the FAA doesn’t know for sure that cellular signals interfere with flight equipment. Theoretically, they could. That was enough for the ban, the FAA figured.

But it’s not all bad news. In fact, you don’t have to stop playing Candy Crush or jamming out to your iPod, at least not according to the FAA (rules differ from airline to airline). In 2013, they ruled that small electronic devices are totally safe during all phases of flight, including takeoff and landing. You just have to make sure they’re in airplane mode. It’s not the devices themselves that could cause problems; it’s the cellular service. Cut ties to the networks and you’re free to keep using your gadgets, as far as the FAA is concerned.

It may be annoying to take a forced break from phone calls, but ask yourself this: Would flights be any more pleasant if half the passengers were squawking on their phones the whole time? Maybe we’re better off with the ban.

In fact, that’s exactly the argument Federal Communications Commission Chairman Ajit Pai made in 2017. A 2013 proposal from Pai’s predecessor, Tom Wheeler, would have lifted the ban on cellphone use during flights. Contemporary flight communications systems aren’t vulnerable to interference from cellular signals, Wheeler argued. There’s no reason people shouldn’t be able to connect during flights.

Pai didn’t disagree with that argument. He just thinks people shouldn’t talk on their phones on airplanes because it’s annoying. He shot down the ban a few months after taking office.

“Taking [the proposal] off the table permanently will be a victory for Americans across the country who, like me, value a moment of quiet at 30,000 feet,” Pai said in The Washington Post.

The failure of the proposal is music to the ears of folks who work in airplane cabins. The Association of Flight-Attendants-CWA, the industry’s labor union, applauded Pai’s decision.

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“The FCC is making the right decision not to pursue lifting the ban on in-flight calls,” the union’s spokesperson, Taylor Garland, told The Washington Post. “The traveling public and crew members do not want voice calls on planes.”

So there’s that.

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