The famous scientist James Watson summarized it this way: The brain boggles the mind! The human brain is truly impressive: It weighs only about three pounds but controls everything a person does, ever has done, and ever will do—physically, intellectually, and emotionally. No computer even comes close to having the brain’s abilities. The brain controls a person’s actions, reactions, and survival functions, such as breathing. It also has the ability to think, remember, process information, and learn new things.

2The brain is one part of the central nervous system—the system that controls all of the body’s activities. The central nervous system is made up of the brain and the spinal cord. The brain is protected by the skull, and the spinal cord runs through vertebrae of the back—the bones that make up the spine. The spinal cord transmits messages between the brain and other parts of the body through nerve cells called neurons. If a person decides to pick up a book from the shelf—a voluntary action—the brain sends that message to the arm and hand through the spinal cord. And if a person touches a hot surface and burns his or her hand—an involuntary action—the nerve cells in the hand send a pain message to the brain through the spinal cord.

3A constant stream of messages travels through the neurons in the spinal cord, at speeds of more than 150 miles per hour. The human brain never stops working, even when a person is asleep. As well as transmitting messages through the spinal cord, neurons transmit messages from one part of the brain to another. There are approximately 85 billion of these cells in the brain alone. Neurons send messages through tiny branch-like structures that connect to other neurons in different parts of the brain, as well as other parts of the body. The points where neurons meet and transmit information to each other are called synapses. Each neuron may be connected to as many as 10,000 other neurons, resulting in more than 100 trillion synapses in a single brain.

4Although a person cannot increase the amount of neurons in his or her brain, learning new things increases the number of synapse connections between them. Learning and education actually change the structure of the human brain. That structure changes every time a person learns, and every time that person has a new thought or memory. The more a person learns the more there is to think about. And the more there is to think about, the more there is to remember. As a result, the connections between neurons get stronger, and the brain is able to function more effectively. It processes, thinks, analyzes, and stores information more quickly and productively than it did before these connections were made. Neurons are just cells, and everything a person knows is the result of the connections between them.

5Scientists have gained a wealth of knowledge about the human brain, but there is a lot they do not yet understand. The neurologist Santiago Ramón y Cajal, for example, compares the brain to a world of unexplored continents with great stretches of unknown territory. Even so, new discoveries continually increase our knowledge of how the brain functions and how people learn.

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ESSENTIAL QUESTION
In what different ways can people be intelligent?