How Your Brain Works

The Three-Pound Universe

Inside your head, behind your eyes and between your ears, sits the most complicated object scientists have ever studied. It weighs only about 1.4 kilograms — less than a bag of sugar — yet it lets you read these words, remember your last birthday, feel happy or nervous, and imagine things that have never existed.

That object is your brain. It is the reason you are you. Every thought you have ever had, every skill you have ever learned, and every dream you have ever dreamed happened inside this soft, wrinkled organ. People sometimes call it the "three-pound universe" because, like the universe, there is more inside it than we can easily imagine.

This little book opens up the brain and shows you how it works — not magic, but a real machine made of living cells, electricity and chemistry. Let's take a look.

Chapter 1: A Machine Made of Cells

Your brain is built from billions of tiny living cells. The most famous are the neurons, sometimes called nerve cells. You have around 86 billion of them. If you tried to count them one per second, it would take you more than two thousand years.

A neuron does not look like an ordinary blob-shaped cell. It has a main body, lots of branching arms called dendrites that receive messages, and one long tail called an axon that sends messages on. Picture a tiny tree with roots at one end and a single long trunk at the other.

Neurons are not the only cells in the brain. Helper cells called glia outnumber them. Glia feed the neurons, clean up waste, and wrap the axons in a fatty coating that helps signals travel faster, a bit like the plastic coating on an electrical wire.

What makes the brain powerful is not just the number of cells but how they are connected. Each neuron may link to thousands of others, creating trillions of connections. This vast web is your brain's wiring, and it is different in every single person.

Chapter 2: Sending the Signal

How does one neuron talk to another? The answer is a clever mix of electricity and chemistry.

When a neuron is switched on, a small electrical pulse races down its axon. This pulse is called a nerve impulse, and it can travel faster than 100 metres per second — quicker than a racing car. But here is the surprising part: neurons do not actually touch each other. Where the axon of one neuron reaches the dendrite of the next, there is a microscopic gap called a synapse.

The electrical signal cannot jump the gap on its own. Instead, the neuron releases tiny bursts of chemicals called neurotransmitters. These float across the synapse and land on the next neuron, telling it to fire its own electrical pulse — or sometimes telling it to stay quiet. In this way, signals ripple through the brain in patterns, like lights flickering across a giant city at night.

Everything you think and feel is, at its heart, billions of these signals passing along this living network.

Chapter 3: A Map of the Brain

The brain is not one single lump. It has different regions, each with its own jobs, working together as a team.

The largest part is the cerebrum, the big wrinkled dome that fills most of your skull. The wrinkles let a huge sheet of brain tissue fold up to fit inside your head. This is where thinking, planning, language and imagination happen. The cerebrum is split into a left half and a right half, joined by a thick bundle of fibres.

Tucked under the back of the cerebrum is the cerebellum, which means "little brain". It keeps your movements smooth and your balance steady. When you ride a bike or catch a ball without thinking, you can thank your cerebellum.

At the base, connecting your brain to your spinal cord, is the brainstem. It runs the jobs you never have to think about — your heartbeat, your breathing, your blood pressure. Deep in the centre sit smaller structures with big roles: the hippocampus, which helps form memories, and the amygdala, which handles strong emotions like fear.

Chapter 4: How You Learn and Remember

One of the most amazing things your brain does is change itself. Every time you learn something new, the connections between your neurons adjust. Connections you use often grow stronger; ones you ignore grow weaker. Scientists call this ability plasticity.

This is exactly why practice works. The first time you try to play a tune, ride a bike or solve a tricky sum, the right neurons fire awkwardly. Repeat it, and those neurons fire together again and again until the pathway becomes smooth and automatic. As the saying among scientists goes, "neurons that fire together, wire together."

Memory comes in different kinds. Short-term memory holds a phone number just long enough to type it in. Long-term memory can store something for a lifetime, like your home address or how to swim. The hippocampus acts like a librarian, deciding which experiences are important enough to file away for the long term. The more an experience matters to you — or the more you revisit it — the stronger that memory becomes.

Chapter 5: The Senses and the Outside World

Your brain sits in total darkness and silence inside your skull. It never sees light or hears a sound directly. Everything you experience reaches it as electrical signals sent from your senses.

When light enters your eyes, special cells turn it into nerve impulses that travel to the back of the brain, where they are turned into the picture you "see". Sound waves shake tiny parts inside your ears, which send signals to be turned into what you "hear". Touch, taste and smell all work the same way — sense organs collect information and pass coded messages to the brain.

In other words, the colourful, noisy, touchable world you experience is really a kind of model built inside your head from millions of incoming signals. Your brain even fills in gaps and makes quick guesses, which is why optical illusions can fool you. They reveal the clever shortcuts your brain takes to make sense of the world quickly.

Chapter 6: Feelings and Emotions

Your brain does not just think — it feels. Emotions like joy, anger, fear and excitement are produced by the brain too, especially by those deep central structures like the amygdala.

Emotions are useful, not just pleasant or unpleasant. Fear can keep you safe by warning you of danger. Happiness encourages you to repeat things that are good for you. Even uncomfortable feelings, like nervousness before a test, are your brain getting your body ready to perform.

The brain and body work closely together here. When you feel scared, your brain signals your body to release chemicals that make your heart pound and your muscles tense, ready to run or fight. Learning to understand and manage these feelings is a real skill, and it grows stronger with practice, just like any other brain ability.

Chapter 7: Sleep and Looking After Your Brain

If the brain is so busy, when does it rest? The answer is surprising: even during sleep, your brain stays remarkably active.

While you sleep, your brain replays the day, strengthening important memories and letting go of the unimportant ones. It also cleans itself, flushing out waste chemicals that build up while you are awake. This is one reason you feel foggy and grumpy after a bad night — your brain has not had a chance to tidy up. Most young people need around nine hours of sleep for their brains to work at their best.

Just like the rest of your body, your brain thrives on good care:

• Sleep well, to lock in learning and reset for the next day.
• Eat healthy food and drink water, because the brain needs steady fuel and is mostly made of water.
• Move and exercise, which sends more oxygen-rich blood to the brain.
• Challenge yourself by learning new things, which builds fresh connections.

Your brain has carried every thought you have ever had and will keep building, changing and growing for your whole life. Understanding it is one of the best ways to take care of it. To keep exploring the machine you live in, read The Human Body: An Owner's Guide, or discover the people who unlocked secrets like these in Great Scientists and Their Discoveries.