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How Does a Vending Machine Know Which Bill You Used?

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While coins are easy for automated systems to process because of their differing sizes and weights, bills are a different animal. They became a challenge for the vending industry once inflation meant that objects which plopped out of machines were worth enough to warrant paper money.

One of the original and still-leading manufacturers of this technology is MEI, which originated in the 1970s within Mars Inc., the candy company that makes M&Ms, Skittles and Twix Bars. (MEI stands for “Mars Electronics International.”) The oldest machines took advantage of the magnetic ink used in paper currency. The machines contained a magnetic head that could “read” the specific qualities of each denomination. (Similar technology reads audio cassette tapes.) Markings, crumples, and fading could distort the magnetic “signature” on a bill, and the heads themselves could be clouded with dust over time. That’s why vending machines so frequently spat back your dollar and left poor souls soda-less in the '80s.

Nowadays, many vending machines are made with photocells or cameras that can be programmed to recognize the visual markings of various bills. Lights illuminate the bill, allowing the tiny camera to inspect for these sometimes subtle markers. This is advantageous because, should the Treasury issue a new design, the manufacturer of the machine can program it into its computer. They are also less susceptible to con artists—counterfeits with magnetic ink that wouldn’t fool a computer or human could easily pass through the magnetic head of an old vending machine.

Many top-of-the-line bill validators used in businesses like Las Vegas casinos use several techniques. They can detect magnetic particles, the unique paper and high-iron ink used by the U.S. Treasury Department, and even unique shadows that appear on each part of a bill when lit from behind.

Since they're not a huge target for big-money fraudsters, your local laundromat or break room machines probably rely on cheap, simple technology to get the job done. It's just one reason why a startup that sent quarters in the mail every two-week laundry cycle never really took off—thanks to automated bill readers, making change at the laundromat is easy enough already.

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Big Questions
What Are Curlers Yelling About?
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WANG ZHAO/AFP/Getty Images

Curling is a sport that prides itself on civility—in fact, one of its key tenets is known as the “Spirit of Curling,” a term that illustrates the respect that the athletes have for both their own teammates and their opponents. But if you’re one of the millions of people who get absorbed by the sport once every four years, you probably noticed one quirk that is decidedly uncivilized: the yelling.

Watch any curling match and you’ll hear skips—or captains—on both sides barking and shouting as the 42-pound stone rumbles down the ice. This isn’t trash talk; it’s strategy. And, of course, curlers have their own jargon, so while their screams won’t make a whole lot of sense to the uninitiated, they could decide whether or not a team will have a spot on the podium once these Olympics are over.

For instance, when you hear a skip shouting “Whoa!” it means he or she needs their teammates to stop sweeping. Shouting “Hard!” means the others need to start sweeping faster. If that’s still not getting the job done, yelling “Hurry hard!” will likely drive the point home: pick up the intensity and sweep with downward pressure. A "Clean!" yell means put a brush on the ice but apply no pressure. This will clear the ice so the stone can glide more easily.

There's no regulation for the shouts, though—curler Erika Brown says she shouts “Right off!” and “Whoa!” to get her teammates to stop sweeping. And when it's time for the team to start sweeping, you might hear "Yes!" or "Sweep!" or "Get on it!" The actual terminology isn't as important as how the phrase is shouted. Curling is a sport predicated on feel, and it’s often the volume and urgency in the skip’s voice (and what shade of red they’re turning) that’s the most important aspect of the shouting.

If you need any more reason to make curling your favorite winter sport, once all that yelling is over and a winner is declared, it's not uncommon for both teams to go out for a round of drinks afterwards (with the winners picking up the tab, obviously). Find out how you can pick up a brush and learn the ins and outs of curling with our beginner's guide.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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Why You Should Never Take Your Shoes Off On an Airplane
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What should be worn during takeoff?

Tony Luna:

If you are a frequent flyer, you may often notice that some passengers like to kick off their shoes the moment they've settled down into their seats.

As an ex-flight attendant, I'm here to tell you that it is a dangerous thing to do. Why?

Besides stinking up the whole cabin, footwear is essential during an airplane emergency, even though it is not part of the flight safety information.

During an emergency, all sorts of debris and unpleasant ground surfaces will block your way toward the exit, as well as outside the aircraft. If your feet aren't properly covered, you'll have a hard time making your way to safety.

Imagine destroying your bare feet as you run down the aisle covered with broken glass, fires, and metal shards. Kind of like John McClane in Die Hard, but worse. Ouch!

Bruce Willis stars in 'Die Hard' (1988)
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A mere couple of seconds delay during an emergency evacuation can be a matter of life and death, especially in an enclosed environment. Not to mention the entire aircraft will likely be engulfed in panic and chaos.

So, the next time you go on a plane trip, please keep your shoes on during takeoff, even if it is uncomfortable.

You can slip on a pair of bathroom slippers if you really need to let your toes breathe. They're pretty useless in a real emergency evacuation, but at least they're better than going barefoot.

This post originally appeared on Quora. Click here to view.

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