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How Does a Plastic Bag Fix a Buggy Credit Card?

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Reader Amanda writes, “I was a cashier for a few years and when the register wouldn’t read a credit card, we would wrap the card in a plastic bag and run it through again and it would work. What makes the register read the card when it’s wrapped with a plastic bag?"

The black stripe on the back side of the credit card is made up of a bunch of tiny magnetic particles bound in plastic. The particles are arranged in magnetic and non-magnetic “zones” to encode the data—like your account number, expiration date, etc.—that the card reader on the register needs to process the transaction. When you swipe the card, the card reader reads the information by detecting the changes between the zones. For the transaction to work, the info needs to get from the card to the reader to the computer without any errors.

The strip is pretty delicate, and the data on it can be corrupted by exposing it to a strong magnet or scratching it. Damage can also occur gradually with use and carrying the card around in your wallet. Over time, some of the magnetic particles can get dragged out of position in a process called smearing. If enough magnetic bits move into a non-magnetic space to create a weak signal, the data gets corrupted and the card reader gets an error.

With just a little bit of magnetic material in them, the contaminated non-magnetic zones still have a much lower magnetic strength than the parts that are supposed to magnetized. Increasing the distance between the card reader and the corrupted zones is enough to get  the reader to read those weak parts as non-magnetized again. A plastic bag, usually at-hand at the cash register, makes a great spacer. Wrapped around the card, the bag basically mutes the corrupted parts of the magnetic stripe so the reader is presented with the data as its supposed to be.

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Big Questions
Why Is the American Flag Displayed Backwards on Military Uniforms?
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In 1968, famed activist Abbie Hoffman decided to crash a meeting of the House Un-American Activities Committee in Washington by showing up in a shirt depicting the American flag. Hoffman was quickly surrounded by police, who ripped his shirt off and arrested him for desecration of the Red, White, and Blue.

Hoffman’s arrest is notable today because, while it might be unpatriotic to some, wearing the American flag, burning it, or otherwise disrespecting it is not a violation of any federal law. In 1989, the Supreme Court ruled that it would be unconstitutional to prosecute any such action. Still, Americans have very fervent and strict attitudes toward displaying the flag, a longstanding symbol of our country’s freedom. According to the U.S. Flag Code, which was first published in 1923, you shouldn’t let the flag touch the ground or hang it upside-down. While there’s no express prohibition about reversing the image, it’s probably a safe bet you shouldn’t do that, either.

Yet branches of the U.S. military are often spotted with a seeming mirror reflection of the flag on their right shoulder. If you look at a member in profile, the canton—the rectangle with the stars—is on the right. Isn’t that backwards? Shouldn’t it look like the flag on the left shoulder?

The American flag appears on a military uniform
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Not really. The flag is actually facing forward, and it’s not an optical illusion.

When a service member marches or walks forward, they assume the position of a flagpole, with the flag sewn on their uniform meant to resemble a flag flapping in the breeze. With the canton on the right, the flag would be fluttering behind them. If it were depicted with the canton on the left, the flag would be flying backward—as though it had been hung by the stripes instead of the stars nearest to the pole. The position of the flag is noted in Army Regulation 670-1, mandating the star field should face forward. The official term for this depiction is “reverse side flag.”

As for Hoffman: His conviction was overturned on appeal. In 1970, while at a flag-themed art show in New York, he was invited to get up and speak. He wore a flag shirt for the occasion.

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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Big Questions
What Causes Sinkholes?
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Mark Ralston/AFP/Getty Images

This week, a sinkhole opened up on the White House lawn—likely the result of excess rainfall on the "legitimate swamp" surrounding the storied building, a geologist told The New York Times. While the event had some suggesting we call for Buffy's help, sinkholes are pretty common. In the past few days alone, cavernous maws in the earth have appeared in Maryland, North Carolina, Tennessee, and of course Florida, home to more sinkholes than any other state.

Sinkholes have gulped down suburban homes, cars, and entire fields in the past. How does the ground just open up like that?

Sinkholes are a simple matter of cause and effect. Urban sinkholes may be directly traced to underground water main breaks or collapsed sewer pipelines, into which city sidewalks crumple in the absence of any structural support. In more rural areas, such catastrophes might be attributed to abandoned mine shafts or salt caverns that can't take the weight anymore. These types of sinkholes are heavily influenced by human action, but most sinkholes are unpredictable, inevitable natural occurrences.

Florida is so prone to sinkholes because it has the misfortune of being built upon a foundation of limestone—solid rock, but the kind that is easily dissolved by acidic rain or groundwater. The karst process, in which the mildly acidic water wears away at fractures in the limestone, leaves empty space where there used to be stone, and even the residue is washed away. Any loose soil, grass, or—for example—luxury condominiums perched atop the hole in the ground aren't left with much support. Just as a house built on a weak foundation is more likely to collapse, the same is true of the ground itself. Gravity eventually takes its toll, aided by natural erosion, and so the hole begins to sink.

About 10 percent of the world's landscape is composed of karst regions. Despite being common, sinkholes' unforeseeable nature serves as proof that the ground beneath our feet may not be as solid as we think.

A version of this story originally ran in 2014.

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