Your brain holds hundreds of billions of nerve cells. These cells come in two flavors: neurons (which get all the attention) and glial cells (which play an essential but often-overlooked supporting role).

Neurons carry electrical signals through your brain, and through the rest of your body. Estimates range, but the most widely cited calculations suggest that you have 100 billion neurons. (If you need an ego boost, compare that with the 300,000 neurons in the brain of the humble fruit fly.) Amazingly, there are at least 10 times as many glial cells, which provide nourishment, protection, waste disposal, speed enhancement (see ), and other support services for the spotlight-hogging neurons.

Here's a look at a single neuron:

Up close and personal, a neuron looks like some form of futuristic vegetation. It receives messages through tree-like branches called dendrites. It then sends an electrical signal down a long tube-like structure called the axon. Add up the cumulative effect of several billion of these electrical impulses and you get a symphony, a treatise on law, or an episode of Buffy the Vampire Slayer.

The real magic happens when an electrical signal reaches the end of a neuron. At this point the neuron releases a bundle of chemicals into a tiny gap called a synapse

Now that you know your brain loves sugar, you might try to overclock it with a steady diet of chocolate, fudge icing, and gummy bears. Not so fast. The problem is that unlike the muscles in your body, your brain can only store the tiniest amount of glucose. Instead, it depends on your body to feed it a constant sugar supply through your blood. And simple, sugary foods don't stick around in your bloodstream for very long.

To understand the problem, consider what happens when you eat a quintuple-chocolate frosted donut:

They may sound like two nasty hobbits, but these two hormones play a key role in shaping your appetite.

Scientists have discovered how the SCN works by putting good-natured people in dark caves for long amounts of time. Not only is this an entertaining way for brain researchers to while away a weekend, it also turns out to be surprisingly informative. Confining people in caves tells us how humans manage their time when they have no external cues to indicate whether it's morning, midnight, or midday.

During cave studies, volunteers are free to sleep whenever they like. However, they gravitate to a 24- to 25-hour cycle that closely resembles what we think of as a normal human day. As this cycle, called the circadian rhythm, draws to its close, the participants get ready to sleep. As the cycle starts again, they pass through their deepest sleep, and then rise to start a new day. The cave studies show that you don't need the rising and setting of the sun to know when to get out of bed. Instead, the SCN keeps an internal clock running all the time.

The circadian rhythm doesn't just govern sleep and wakefulness. It also influences a host of body processes that vary over the course of a day. For example, body temperature is at its lowest in the early morning, and it peaks in the evening. Similarly, rote memory (stuff you memorize by repeating over and over) is keenest before lunch, and coordination is best in the afternoon (around 2:00 p.m.). These daily schedules are controlled by an intricate family of hormones. These schedules also affect diseases and chronic conditions. For example, the early morning tends to be the most difficult time for people with rheumatoid arthritis and asthma sufferers (along with the late evening). It's also a risky time for heart attacks.

Although you undoubtedly remember a few of your most memorable dreams, you probably don't have as good an idea about the overall pattern of your dreaming, and how your dreams compare to the nightly visions of other people. Large dream studies shed some light into these questions by comparing the dream journals of hundreds of volunteers, sometimes over long periods of time. Here are some of their discoveries:

Along with distortions of shape, your brain can also mislead you when sizing up the length, size, and color of an object. And when the brain's assumptions fail, the effects tell us quite a bit about the brain's book of visual rules, tricks, and shortcuts.

As you learned in , it's relatively rare for new neurons to appear in your brain. However, the structure of your brain is being continuously reworked. Synapses—the connections between neurons—are constantly being strengthened or weakened, and new dendrites are growing to link neurons together in new patterns. This continuous process of brain reorganization underlies all long-term memory and learning.

Although memories are scattered throughout your brain, there is one brain region that plays a key role in coordinating memory storage: the hippocampus. The hippocampus is a small neuron bundle that's buried near the bottom of the brain. The human brain has two of them, one on the left side and one on the right.

The next time you're searching for your keys, grasping for a name, or lost in the mall, here's something to think about. The odds are that you haven't forgotten the information you need. Rather, it probably never entered your memory in the first place.

Studies consistently show that people don't bother to remember anything that doesn't scream out its importance. Consider some of the objects that decorate your daily life. Could you draw the pattern on your favorite coffee mug? Can you describe the clerk where you bought your last chocolate bar? Do you remember what your wife was wearing when you last saw her?

From birth, your brain comes pre-wired with a few key emotional responses, like pleasure and fear. These emotional responses are a fundamental part of the human condition—in other words, even someone from the most isolated tribe in the South Pacific has exactly the same emotional programming as you do. They might describe their emotions differently, and they might apply them differently (for example, they might not find much to fear in a family of cockroaches, or they might devoutly worship our discarded bottles of Coca-Cola) but they'll still experience the full gamut of human emotion in their quite different lives.

Some of the best evidence supporting this argument is found in cross-cultural studies comparing human facial expressions. When shown pictures of people from a remote culture, we have no trouble identifying surprise, anger, fear, disgust, grief, and the rest of the lexicon of human emotion. Similarly, although those South Pacificans can't make sense of our laptops, smart phones, and iPods, they have no trouble interpreting the look on our face when one of these devices conks out on a Micronesian island miles from civilization.

Now for the bad news. Not only is the brain designed to give you pleasure, it's also designed to hold it back. Here's why:

In , you learned about the curious phenomenon of blindsight (), where part of the brain is able to react to something even though it isn't consciously perceived. Blindsight shows that there's more than one pathway in your brain that responds to the things you see and hear.

Constant stress is like having a car alarm going off in your body around the clock. Eventually, you'll learn to tune out the cacophony. However, you'll still end up with a wicked headache at the end of the day.

When your brain feels threatened for long periods of time, your body experiences the following changes:

In , you learned about the set point theory, which suggests the body uses every trick in the book to maintain its current weight. The depressing conclusion is that if your weight inches up over the years, you'll have a hard time fighting it back down.

The set point theory is just one example of homeostasis, and many researchers suggest happiness is another. To understand this theory, it's important to distinguish between pleasure (raw, physical feel-good feelings) and happiness (the more ambiguous state of contentment and optimism that we all generally strive for). Happiness is probably a secondary emotion that's generated in our deep-thinking cerebral cortex. In other words, pleasure is the biological drive that rewards our actions, while happiness is the subjective state we enter into when the conscious part of our brain reflects on our pleasure.

Here's the problem. According to the set point theory, our level of happiness is a basic personality trait. And much as your body fights to get back to its set point weight, your brain always drifts towards its set point of happiness. Some people are always cheery, no matter what apparent tragedies befall them. These are the people who don't mind being confined to a bed with kidney stones because it gives them a chance to catch up on their crossword puzzles. Other people study the dark lining in the happiest-seeming events. They worry about the tax implications of winning the lottery. Most people fall somewhere in between, and have brains that prefer a more moderate balance of moderate worry and mild satisfaction.

The prefrontal cortex is the portion of your brain that sits at the very front, just above your eyes and behind your forehead.

You already met your prefrontal cortex in , where you learned how it plays a key role in motivation. The prefrontal cortex also crams in a range of high-level mental processes. For this reason, it's often called the brain's executive center (presumably by people who actually believe executives do more than dine out on power lunches).

Here are some of the tasks that the prefrontal cortex takes on:

Your brain doesn't like to waffle. Rather than mulling a situation over, people prefer to make quick, provisional decisions, and then tweak these decisions with minor adjustments.

Today is a special day in Ted's life. After a protracted struggle with a nasty smoking addiction, he's finally decided to ditch the habit. Not one to let the moment pass, he immediately sets off to his local pharmacy to buy a nicotine patch and is promptly run over by an oil tanker.

The question is this: was it a good idea for Ted to quit smoking? Clearly it was a bad move—in fact, it's no exaggeration to state that Ted's decision to kick the habit resulted in his untimely demise. But this odd turn of events should bring little cheer to Joe Camel, as it doesn't make the slightest difference in the overall statistical conclusion that, all things considered, smoking is a surefire way to end up sicker and die sooner. Ted's life is just one data point in a huge mass of information.

As you've already seen, your brain has a deep hunger for certainty. It's most comfortable with concrete, actionable information, and it barely tolerates ambiguity. Rather than use logic to openly investigate an issue, your brain prefers to latch onto a conclusion instinctively and use logic after the fact to defend it. The colorful writer Edward de Bono describes it this way: "The natural tendency of thinking is to support a view arrived at by other means."

To battle this tendency, you need to master the art of suspending judgment. The longer the interval between the time a question is posed and the time your brain locks into an answer, the more objective you'll be. Once your brain forms an expectation, that expectation acts like a magnet, pulling all your thinking and reasoning in one direction.

In some cases, you'll need to accept the ambiguity of having no clear answer at all. To do so, you need to fight against your brain's instincts, which favor bad explanations to no explanation. This tendency underlies everything from wrongful convictions to wacky superstitions.

The most important step in critical thinking isn't applying logic in a particular way, but establishing the right foundation. You need to create an environment that leaves space for your brain to think.

As you've just seen, part of the trick is preventing your brain from making any preliminary conclusions. Another key point is to recognize your own fallibility. Although you can't erase your personal biases, you can keep them in check by adopting a mindset of intellectual humility. Here are some points to help:

The most creative problem solving begins when you challenge an existing assumption with a new idea. Unfortunately, it's all too easy to rule out an apparently absurd new direction before it gets off the ground. To check this habit, you need to master the art of provocation.

One technique invented by the creative thinker Edward de Bono is to use the word po to signal that a new idea is a provocation that can't be judged, but has to be used as a springboard for new ideas. For example, imagine you're trying to use creative thinking to figure out how to attract new customers to a faltering restaurant. You might use po like this:

These ideas are obviously illogical. But when you're forced to grapple with them, you might arrive at the following new ideas:

Po: Let's admit we suck.

Long loved by high school counselors, the typical career test clarifies absolutely nothing. Career tests are famous for brilliant pronouncements like this: Your nurturing side suggests you'd make a fine doctor, veterinarian, or housewife. Your need for order suggests you'd enjoy life as a judge, accountant, or sanitation worker.

The problem with mapping personalities to careers is that most careers have room for a range of different roles. Although some professions require extreme personality types (for example, the prospects are dim for introverted break dancers and low-conscientiousness house cleaners), most are surprisingly flexible. For example, it's obvious that the self-directed focus that's needed for a film critic, computer programmer, or research scientist makes these good positions for introverts. However, extroverts will be just as happy if they have the freedom to express their social side—say, interviewing actors and discussing movies at a film festival (movie critic), leading a team of code warriors into battle (programmer), or collaborating with colleagues and presenting new techniques and ideas (scientist). Personality analysis reveals that all these professions are more commonly occupied by introverts, but that fails to account for the perfectly happy extroverts who have carved out a slightly different niche in the same world. Similarly, job satisfaction also relies on many other factors, such as the type of work you're doing, its rewards (in money and prestige), your time commitment, the attitude of the people you work with, and how high your boss scores on the neuroticism scale.

Scientists know a fair bit about what makes new people into little boys and girls. It all starts with the compact spools of DNA called chromosomes. As a human, you have 23 pairs of chromosomes curled up in virtually every cell of your body. (That's 46 chromosomes in each cell, for those of you who nodded off in math class.) If you could uncurl your chromosomes and lay them out on a tabletop, they'd look like the figure shown here. (Assuming you're a guy. If you're a woman, you have two copies of the X chromosome at the bottom-right of the figure, and no Y chromosome.)

When two people get together in the interest of DNA sharing, they pass along one chromosome out of each pair. If you're a woman, these chromosomes are packaged up in an egg. If you're a man, they're carried along by your trusty swimmer, the sperm. When a sperm and an egg cell meet, this combination of chromosomes is reassembled into the 23 pairs needed to produce an entire human being, complete with body, brain, and adult-onset neuroses.

Examining the brain in love is relatively new ground for neuroscience. However, a few love studies provide tempting clues.

In 1999, a study concluded that falling in love is rather like acquiring a psychiatric illness. The study tested a group of college students who had recently fallen in love, but hadn't consummated their new passion. These new lovers had significantly lower than normal levels of serotonin, a key neurotransmitter that plays a host of subtle roles in the brain. In fact, their sagging serotonin levels were in line with sufferers of OCD (

Wiring a brain is somewhat like sculpting a statue. You begin with more than enough stone (in the form of excess neurons before birth and excess synapses during childhood). The craft lies in chiseling away the excess until you're left with the form you want.

The figure shown here compares the connections between neurons from birth until two years. The number of neurons doesn't change. However, as the child develops, each neuron sends out a thicket of dendrites in search of other neurons. It's a bit like a lonely partygoer hunting for friends to talk to.

After 24 months, this wave of synaptic growth reaches its peak, leaving a heavily-connected brain and an emotionally unpredictable two-year old. This is when synaptic pruning ramps up. Frequently used connections strengthen, while neglected ones gradually shrivel away. This is one of the reasons that newborn babies can distinguish between more speech sounds than adults, teenagers, and even one-year-olds. As babies master a language, they stop paying attention to the sounds that aren't important, and those connections are trimmed away.

Estimates suggest that the baby brain loses as many as 100,000 synapses each second at the height of its development. As an adult, your brain retains little more than half of the synapses you had as a two-year-old.

To describe how strongly a specific characteristic depends on your genetic makeup, scientists use a measurement called heritability, which ranges from 0 to 1.

A heritability of 0 means the variance in a trait is entirely due to environmental factors. For example, language has a heritability of 0—if you speak English and your dentist speaks Hindi, it's because you were raised in different cultures.

A heritability of 1 means the variance in a trait is entirely up to the genes. For example, your blood type has a heritability of 1—it depends on your parents, not the unwritten rules of society.

Your height is obviously a bit more complex. The link between genes and height varies throughout your life but is strongest at adulthood, when the heritability sits at about 0.8. In other words, if you gather a group of people and measure their heights, about 800f the variance can be explained by genetics. This is a high heritability, which makes a strong argument for all-in-the-family basketball picnics.