Why do you care how a customer reacts to a purchase? Good question. By understanding post-purchase behavior, you can understand the influence and the likelihood of whether a buyer will repurchase the product (and whether she will keep it or return it). You'll also determine whether the buyer will encourage others to purchase the product from you. Satisfied customers can become unpaid ambassadors for your business, so customer satisfaction should be on the top of your to-do list. People tend to believe the opinions of people they know. People trust friends over advertisements any day. They know that advertisements are paid to tell the "good side" and that they're used to persuade them to purchase products and services. By continually monitoring your customer's satisfaction after the sale, you have the ability to avoid negative word-of-mouth advertising.

Few people will be surprised to hear that poverty tends to create stress: a 2006 study published in the American journal Psychosomatic Medicine, for example, noted that a lower socioeconomic status was associated with higher levels of stress hormones in the body. However, richer economies have their own distinct stresses. The key issue is time pressure. A 1999 study of 31 countries by American psychologist Robert Levine and Canadian psychologist Ara Norenzayan found that wealthier, more industrialized nations had a faster pace of life—which led to a higher standard of living, but at the same time left the population feeling a constant sense of urgency, as well as being more prone to heart disease. In effect, fast-paced productivity creates wealth, but it also leads people to feel time-poor when they lack the time to relax and enjoy themselves.

Individuals who perform at a high level in their profession often have instant credibility with others. People admire them, they want to be like them, and they feel connected to them. When they speak, others listen—even if the area of their skill has nothing to do with the advice they give. Think about a world-famous basketball player. He has made more money from endorsements than he ever did playing basketball. Is it because of his knowledge of the products he endorses? No. It's because of what he can do with a basketball. The same can be said of an Olympic medalist swimmer. People listen to him because of what he can do in the pool. And when an actor tells us we should drive a certain car, we don't listen because of his expertise on engines. We listen because we admire his talent. Excellence connects. If you possess a high level of ability in an area, others may desire to connect with you because of it.

Think of the brain as a city. If you were to look out over a city and ask "where is the economy located?" you'd see there's no good answer to the question. Instead, the economy emerges from the interaction of all the elements—from the stores and the banks to the merchants and the customers. And so it is with the brain's operation: it doesn't happen in one spot. Just as in a city, no neighborhood of the brain operates in isolation. In brains and in cities, everything emerges from the interaction between residents, at all scales, locally and distantly. Just as trains bring materials and textiles into a city, which become processed into the economy, so the raw electrochemical signals from sensory organs are transported along superhighways of neurons. There the signals undergo processing and transformation into our conscious reality.

Someone else's body language affects our own body, which then creates an emotional echo that makes us feel accordingly. As Louis Armstrong sang, "When you're smiling, the whole world smiles with you." If copying another's smile makes us feel happy, the emotion of the smiler has been transmitted via our body. Strange as it may sound, this theory states that emotions arise from our bodies. For example, our mood can be improved by simply lifting up the corners of our mouth. If people are asked to bite down on a pencil lengthwise, taking care not to let the pencil touch their lips (thus forcing the mouth into a smile-like shape), they judge cartoons funnier than if they have been asked to frown. The primacy of the body is sometimes summarized in the phrase "I must be afraid, because I'm running."

Restricting the number of items customers can buy boosts sales. Brian Wansink, Professor of Marketing at Cornell University, investigated the effectiveness of this tactic in 1998. He persuaded three supermarkets in Sioux City, Iowa, to offer Campbell's soup at a small discount: 79 cents rather than 89 cents. The discounted soup was sold in one of three conditions: a control, where there was no limit on the volume of purchases, or two tests, where customers were limited to either four or twelve cans. In the unlimited condition shoppers bought 3.3 cans on average, whereas in the scarce condition, when there was a limit, they bought 5.3 on average. This suggests scarcity encourages sales. The findings are particularly strong because the test took place in a supermarket with genuine shoppers. It didn't rely on claimed data, nor was it held in a laboratory where consumers might behave differently.

Although technology has the potential to increase productivity, it can also have a negative impact on productivity. For example, in many office environments workers sit at desks with computers and have access to the internet. They are able to check their personal e-mails and use social media whenever they want to. This can stop them from doing their work and make them less productive. Introducing new technology can also have a negative impact on production when it causes a change to the production process or requires workers to learn a new system. Learning to use new technology can be time consuming and stressful for workers and this can cause a decline in productivity.

Up until about 6,000 years ago, most people were farmers. Many lived in different places throughout the year, hunting for food or moving their livestock to areas with enough food. There was no need to tell the time because life depended on natural cycles, such as the changing seasons or sunrise and sunset. Gradually more people started to live in larger settlements, and some needed to tell the time. For example, priests wanted to know when to carry out religious ceremonies. This was when people first invented clocks—devices that show, measure, and keep track of passing time. Clocks have been important ever since. Today, clocks are used for important things such as setting busy airport timetables—if the time is incorrect, aeroplanes might crash into each other when taking off or landing!

Managers are always looking for ways to increase productivity, which is the ratio of costs to output in production. Adam Smith, writing when the manufacturing industry was new, described a way that production could be made more efficient, known as the "division of labor." Making most manufactured goods involves several different processes using different skills. Smith's example was the manufacture of pins: the wire is straightened, sharpened, a head is put on, and then it is polished. One worker could do all these tasks, and make 20 pins in a day. But this work can be divided into its separate processes, with a number of workers each performing one task. Because each worker specializes in one job, he or she can work much faster without changing from one task to another. Now 10 workers can produce thousands of pins in a day—a huge increase in productivity from the 200 they would have produced before.

Sometimes the pace of change is far slower. The face you saw reflected in your mirror this morning probably appeared no different from the face you saw the day before—or a week or a month ago. Yet we know that the face that stares back at us from the glass is not the same, cannot be the same, as it was 10 minutes ago. The proof is in your photo album: Look at a photograph taken of yourself 5 or 10 years ago and you see clear differences between the face in the snapshot and the face in your mirror. If you lived in a world without mirrors for a year and then saw your reflection, you might be surprised by the change. After an interval of 10 years without seeing yourself, you might not at first recognize the person peering from the mirror. Even something as basic as our own face changes from moment to moment.