The word "cool" has been cool for a long time. Originally associated with temperature, by the 16th century the term had evolved to describe not just the atmosphere, but also an internal state of calm, almost icy composure. And by the late 1800s it began to signify style and hipness and some of the other meanings with which it is associated today. Now, cool is used as a synonym for almost anything good. Music can be cool and restaurants can be cool. Every so often even a minivan seems cool. But not all words and phrases persist. In the 1940s, dress snappy and someone might say you looked "spiffy." In the 1950s, people might say you looked "swell." These days, teenagers might say you're "on fleek." What was once "awesome" is now "dope." Tell someone today that they look spiffy and people will think that you are caught in a time warp. Language is constantly evolving. Certain words and phrases catch on and become popular while others die out and wither away. So what leads some phrases to become more successful than others? Why do some stand the test of time while others die out? There is no record of every time someone utters a certain word or phrase, so to study these questions, a colleague of mine and I turned to the next best thing: books. For hundreds of years, of course, books have documented the words and phrases used to express different ideas. This includes everything from Shakespeare's sonnets to Jane Austen's description of the landed gentry, and many thousands of works by unknown authors in between. Books provide a written record of culture, a constantly evolving collection of snapshots of what things were like across time and space. Using a searchable database of more than five million books from the last 200 years, we were able to track the popularity of thousands of words and phrases over time. Interestingly, we discovered that our senses (e.g., sight, smell and touch) have a big impact on linguistic success. There are multiple ways to convey the same thing, and phrases with similar meanings often act as substitutes, competing for usage. A not-so-friendly person, for example, can be described as unfriendly or cold. An intelligent student can be described as smart or bright. For each of these pairs, one of the phrases relates to the senses (i.e., cold person or bright student) while its semantic analogue (unfriendly person or smart student) does not. While this might seem like a minor difference, it actually has a big impact on linguistic success. Compared with their semantic equivalents (e.g., unfriendly person or smart student), we found that phrases that relate to senses in metaphoric ways (e.g., cold person or bright student) became more popular over time.

According to the UN, developed countries throw away around 30% to 40% of all food purchased. And if food waste was cut by a quarter, world famine could be solved. In the UK, of the 41 million tons of food that is bought each year, 15 million tons are wasted. You might think supermarkets are the biggest culprits, but the truth is that most have made major strides in recent years. One UK waste-advisory charity's best estimate is that supermarket waste accounts for less than 2% of what gets chucked out each year. Part of that is attributable to advances in supply-chain technology. As you might guess, fresh food and short shelf-life products account for a lot of what gets thrown away. But these days good demand-forecasting and inventory-planning software can handle even the trickiest items. Supermarkets have an interest in avoiding waste because margins on fresh produce tend to be quite tight. If you make 25 pence for every £1 ($1.50) of broccoli sold you have to sell three pieces to make up for the loss from one gone bad. So if you have noticed fewer items with reduced stickers, it is because they are getting a grip on the problem. What waste remains is at least partly driven by consumers expecting fresh food items to be constantly available and stacked in attractive displays — both factors pushing food retailers to order more than they can sell. The biggest contributor to Britain's food-waste shame is household rubbish, which in the UK accounts for almost half the food thrown away. Many of us make bad decisions about food, especially when we are hungry, over-ordering in restaurants and over-buying in shops. The most primitive parts of our brains, faced with feast, react as though famine were just around the corner. And yet the game seems to be stacked against consumers. Supermarkets may strive to eliminate spoilage while food is in their supply chain, but once you have paid for something it is not their problem. They would argue, not unfairly, that they have tried to ensure the food you buy is as fresh as possible to give you the best possible chance to consume it before it goes bad. Back in 2010, one British supermarket giant even briefly experimented with a "buy one, get one free later" scheme to help reduce waste. But for the most part, food retail is structured and incentivised to get us to buy as much as possible, regardless of whether we actually need it. In the developing world, anywhere from 6% to 15% of food gets thrown out despite poorer infrastructure, less reliable logistics, hotter climates, and inferior refrigeration. Indeed, weight for weight, in places such as sub-Saharan Africa and south and southeast Asia, people waste only around a tenth of what the British do. The overwhelming differentiator seems to be the value that we attach to food. Having a full fridge to cater to our every whim and those of our families seems more important than not having a full rubbish bin.

Philosophers, psychologists, and scientists have been puzzling over the essential definition of human uniqueness since the beginning of recorded history. The Harvard psychologist Daniel Gilbert says that every psychologist must, at some point in his or her career, write a version of what he calls "The Sentence." Specifically, The Sentence reads like this: The human being is the only animal that ______. The story of humans' sense of self is, you might say, the story of failed, debunked versions of The Sentence. Except now it's not just the animals that we're worried about. We once thought humans were unique for using language, but this seems less certain each year; we once thought humans were unique for using tools, but this claim also erodes with ongoing animal-behavior research; we once thought humans were unique for being able to do mathematics, and now we can barely imagine being able to do what our calculators can. We might ask ourselves: Is it appropriate to allow our definition of our own uniqueness to be, in some sense, reactive to the advancing front of technology? And why is it that we are so compelled to feel unique in the first place? "Sometimes it seems," says Douglas Hofstadter, a Pulitzer Prize–winning cognitive scientist, "as though each new step towards artificial intelligence, rather than producing something which everyone agrees is real intelligence, merely reveals what real intelligence is not." While at first this seems a consoling position — one that keeps our unique claim to thought intact — it does bear the uncomfortable appearance of a gradual retreat, like a medieval army withdrawing from the castle to the keep. But the retreat can't continue indefinitely. Consider: if everything that we thought hinged on thinking turns out to not involve it, then … what is thinking? It would seem to reduce to either an epiphenomenon — a kind of "exhaust" thrown off by the brain — or, worse, an illusion. Where is the keep of our selfhood? The story of the 21st century will be, in part, the story of the drawing and redrawing of these battle lines, the story of human beings trying to stake a claim on shifting ground, flanked by beast and machine, pinned between meat and math. Is this retreat a good thing or a bad thing? For instance, does the fact that computers are so good at mathematics in some sense take away an arena of human activity, or does it free us from having to do a nonhuman activity, liberating us into a more human life? The latter view seems to be more appealing, but less so when we begin to imagine a point in the future when the number of "human activities" left for us to be "liberated" into has grown uncomfortably small. What then?