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Are Carrots Really That Good For Your Eyes?

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When I was a kid, my parents often tried to sell me on the idea that carrots were good for my eyes—and if I wanted to avoid vision correction in the future, I would eat them now. But after I was fitted for my first pair of glasses in fourth grade, they dropped that line and served carrots with a side of ranch dressing and buffalo wings. As I’ve mentioned before, my mom was not exactly scientifically rigorous in her parenting, so I can’t help but wonder if there’s any truth to what she was telling me about carrots and vision.

It seems that Mom was just parroting a commonly held belief that got its start as a bit of wartime propaganda.

During World War II, German planes frequently made bombing runs over Great Britain. In the early 1940s, the British set up a chain of radar stations along the southern coast of England so German bombers could be detected and shot down before they reached land. Not wanting the Germans to know that they had this technology, the British intelligence service began a propaganda campaign focused on the incredible eyesight of the soldiers manning the defenses—including Flight Lieutenant John Cunningham, an RAF fighter pilot dubbed "Cat's Eyes" for his ability to spot bombers in the dead of night.

Cat's Eyes' amazing vision was chalked up to his carrot-heavy diet. The campaign spilled over to the Ministry of Food, which produced informative cooking pamphlets on carrots and other root vegetables, often featuring a character called Dr. Carrot and the slogan “Carrots keep you healthy and help you see in the blackout.” The Germans, British civilians, and parents all over the world bought the story and repeated it endlessly for decades to come, which helped conceal the real reason behind the RAF’s success—and got kids to eat their veggies.

The British propaganda lent carrots a little more credit than they deserve. They won’t turn you into the second coming of Cat's Eyes, but carrots are good for overall eye health because they're rich in beta-carotene, a carotenoid pigment that’s an important precursor for vitamin A. Extreme vitamin A deficiency can lead to night blindness and other eye problems—a big issue in the developing world. For most people in developed countries, though, vitamin A intake is sufficient with a well-rounded diet. For people with no vitamin A-related problems, binging on carrots won’t really improve your vision, though I think my mom will probably gloss over this fact when trying to feed my nephew.

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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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Big Questions
How Are Speed Limits Set?
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When driving down a road where speed limits are oppressively low, or high enough to let drivers get away with reckless behavior, it's easy to blame the government for getting it wrong. But you and your fellow drivers play a bigger a role in determining speed limits than you might think.

Before cities can come up with speed limit figures, they first need to look at how fast motorists drive down certain roads when there are no limitations. According to The Sacramento Bee, officials conduct speed surveys on two types of roads: arterial roads (typically four-lane highways) and collector streets (two-lane roads connecting residential areas to arterials). Once the data has been collected, they toss out the fastest 15 percent of drivers. The thinking is that this group is probably going faster than what's safe and isn't representative of the average driver. The sweet spot, according to the state, is the 85th percentile: Drivers in this group are thought to occupy the Goldilocks zone of safety and efficiency.

Officials use whatever speed falls in the 85th percentile to set limits for that street, but they do have some wiggle room. If the average speed is 33 mph, for example, they’d normally round up to 35 or down to 30 to reach the nearest 5-mph increment. Whether they decide to make the number higher or lower depends on other information they know about that area. If there’s a risky turn, they might decide to round down and keep drivers on the slow side.

A road’s crash rate also comes into play: If the number of collisions per million miles traveled for that stretch of road is higher than average, officials might lower the speed limit regardless of the 85th percentile rule. Roads that have a history of accidents might also warrant a special signal or sign to reinforce the new speed limit.

For other types of roads, setting speed limits is more of a cut-and-dry process. Streets that run through school zones, business districts, and residential areas are all assigned standard speed limits that are much lower than what drivers might hit if given free rein.

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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