Your brain helps you think, feel, learn and move. Your brain keeps your heart beating and your lungs breathing without you thinking about it. Your brain is inside your head. A hard skull protects it. A grown-up's brain weighs about 3 pounds. Your brain is only about 2 percent of your body weight. Your brain uses about 20 percent of your body's energy.
The biggest part of the brain, the cerebrum, has a left half and a right half. A thick band of nerve fibers, the corpus callosum, carries messages between the brain's two hemispheres. The idea that some people are left-brained and others right-brained is a myth. The idea that we use only 10 percent of our brain is a myth. The wrinkly outside of the cerebrum is called the cortex. The folds in the cortex give the brain more surface area.
As of November 2026, a human brain has about 86 billion neurons. Nerve cells called neurons send messages all over your body. Neurons pass messages along using tiny electrical signals. At a synapse, one neuron releases chemicals that cross a tiny gap to the next neuron. Myelin is a fatty coating wrapped around the axons of many neurons. Motor neurons carry messages from the brain to the body, telling muscles when to move.
On September 13, 1848, a railroad crew member named Phineas Gage was hurt when blasting rock sent an iron rod through his head. The iron rod that hit Phineas Gage was about 3 and a half feet (43 inches) long and weighed 13 pounds. Phineas Gage survived the accident, but he lost the sight in his left eye. Phineas Gage lived about twelve years after his accident.
The hippocampus helps the brain form long-term memories. When you learn, your neurons form new connections. The brain's ability to change with experience and learning is called neuroplasticity. The brain can change at any age, though change is a bit easier when you are younger. Parts of the brain keep maturing until people are in their 20s.
Words to know
- synapse: gap between neurons
- myelin: fatty coating
- cortex: wrinkly outside
- neuroplasticity: ability to change
