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What’s the Difference Between In- and Un-?

English has two different prefixes that make a word into its opposite. OK, yes, there are more than two (dis-, a-, anti-, de-, etc.), but in- and un- are the most common. They bring the sense of “not” to an adjective, and they cause trouble because it is often not clear which one should be used for a particular word. Many pairs of in-/un- words are interchangeable. For example, “inalienable” and “unalienable” are both correct and mean the same thing (even the drafters of the Declaration of Independence went back and forth on that one), as do “inadvisable” and “unadvisable.” Still, the two prefixes are not equivalent.

As a pretty flimsy general rule, un- goes with Germanic roots and in- goes with Latin roots, as seen in these pairs: unfriendly, inamicable; unteachable; ineducable; unbelievable, incredible. Still, just because a word has a Latin root doesn’t mean it can’t go with un-: see unproductive, unfortunate, unreliable, undesirable, unconscious…and so on. Un- is also usually found with adjectives formed from participles ending in -ed or -ing: undomesticated, undeveloped, undisciplined, unconcerning, uncomprehending.

On the other hand, if a word has a Germanic root, it pretty much does mean it can’t go with in-. If you do find such a word, it is probably an example of a completely different prefix in-, meaning in or towards (incoming, infield, indwell). In- is much more restricted than un-. Un- is freely productive; it can apply to new words (“this haircut is brand new and unselfied!”), while in- remains frozen in the existing vocabulary, a Latin dinosaur bone.

Un- can even apply to words that already take in-, though when it does it often creates a different, less specific meaning. For example, while the word “indigestible” can be traced back to the meaning “not able to be digested” it carries extra layers of connotation –food that offends the senses or makes you feel bad, information that is too confusing to process– that “undigestible” doesn’t have. “Undigestible” is more straightforwardly “not able to be digested.” A poorly prepared lasagna is indigestible, but a rock is undigestible. Its meaning is composed of its two parts, while the meaning of “indigestible” comes from its long history of use.

But the search for these kinds of meaning difference can quickly turn messy and confusing. Once you start thinking about this too much, in- and un- words start to switch back and forth in your mind like a duck/rabbit optical illusion. Shades of difference in meaning emerge only to dissolve under closer scrutiny. Inaccessible, unaccessible? Inconsolable, unconsolable? Indescribable, undescribable? Surely they mean different things. No, maybe not. Many of these kinds of pairs have been switching back and forth for centuries. (At the current time, the in- forms of these particular words are considered more acceptable.) Some of them have gotten stuck on one setting or the other, and some will continue to be indecisive, or, if you will, undecided.

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Big Questions
What Makes a Cat's Tail Puff Up When It's Scared?
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Cats wear their emotions on their tails, not their sleeves. They tap their fluffy rear appendages during relaxing naps, thrash them while tense, and hold them stiff and aloft when they’re feeling aggressive, among other behaviors. And in some scary situations (like, say, being surprised by a cucumber), a cat’s tail will actually expand, puffing up to nearly twice its volume as its owner hisses, arches its back, and flattens its ears. What does a super-sized tail signify, and how does it occur naturally without help from hairspray?

Cats with puffed tails are “basically trying to make themselves look as big as possible, and that’s because they detect a threat in the environment," Dr. Mikel Delgado, a certified cat behavior consultant who studied animal behavior and human-pet relationships as a PhD student at the University of California, Berkeley, tells Mental Floss. The “threat” in question can be as major as an approaching dog or as minor as an unexpected noise. Even if a cat isn't technically in any real danger, it's still biologically wired to spring to the offensive at a moment’s notice, as it's "not quite at the top of the food chain,” Delgado says. And a big tail is reflexive feline body language for “I’m big and scary, and you wouldn't want to mess with me,” she adds.

A cat’s tail puffs when muscles in its skin (where the hair base is) contract in response to hormone signals from the stress/fight or flight system, or sympathetic nervous system. Occasionally, the hairs on a cat’s back will also puff up along with the tail. That said, not all cats swell up when a startling situation strikes. “I’ve seen some cats that seem unflappable, and they never get poofed up,” Delgado says. “My cats get puffed up pretty easily.”

In addition to cats, other animals also experience piloerection, as this phenomenon is technically called. For example, “some birds puff up when they're encountering an enemy or a threat,” Delgado says. “I think it is a universal response among animals to try to get themselves out of a [potentially dangerous] situation. Really, the idea is that you don't have to fight because if you fight, you might lose an ear or you might get an injury that could be fatal. For most animals, they’re trying to figure out how to scare another animal off without actually going fisticuffs.” In other words, hiss softly, but carry a big tail.

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What Happened to the Physical Copy of the 'I Have a Dream' Speech?
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On August 28, 1963, Martin Luther King Jr. stood on the steps of the Lincoln Memorial and gave a speech for the ages, delivering the oratorical masterpiece "I Have a Dream" to nearly 250,000 people.

When he was done, King stepped away from the podium, folded his speech, and found himself standing in front of George Raveling, a former Villanova basketball player who, along with his friend Warren Wilson, had been asked to provide extra security around Dr. King while he was speaking. "We were both tall, gangly guys," Raveling told TIME in 2003. "We didn't know what we were doing but we certainly made for a good appearance."

Moved by the speech, Raveling saw the folded papers in King’s hands and asked if he could have them. King gave the young volunteer the speech without hesitation, and that was that.

“At no time do I remember thinking, ‘Wow, we got this historic document,’” Raveling told Sports Illustrated in 2015. Not realizing he was holding what would become an important piece of history in his hands, Raveling went home and stuck the three sheets of paper into a Harry Truman biography for safekeeping. They sat there for nearly two decades while Raveling developed an impressive career coaching NCAA men’s basketball.

In 1984, he had recently taken over as the head coach at the University of Iowa and was chatting with Bob Denney of the Cedar Rapids Gazette when Denney brought up the March on Washington. That's when Raveling dropped the bomb: “You know, I’ve got a copy of that speech," he said, and dug it out of the Truman book. After writing an article about Raveling's connection, the reporter had the speech professionally framed for the coach.

Though he displayed the framed speech in his house for a few years, Raveling began to realize the value of the piece and moved it to a bank vault in Los Angeles. Though he has received offers for King’s speech—one collector wanted to purchase the speech for $3 million in 2014—Raveling has turned them all down. He has been in talks with various museums and universities and hopes to put the speech on display in the future, but for now, he cherishes having it in his possession.

“That to me is something I’ll always be able to look back and say I was there,” Raveling said in the original Cedar Rapids Gazette article. “And not only out there in that arena of people, but to be within touching distance of him. That’s like when you’re 80 or 90 years old you can look back and say ‘I was in touching distance of Abraham Lincoln when he made the Gettysburg Address.’"

“I have no idea why I even asked him for the speech,” Raveling, now CEO of Coaching for Success, has said. “But I’m sure glad that I did.”

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