The brain is the complex organ inside your skull that controls everything you think, feel, remember, and do. It's the command center for your entire body, sending signals through your nerves to make your heart beat, your lungs breathe, and your muscles move. When you solve a math problem, remember your best friend's birthday, or feel excited about an upcoming trip, that's your brain at work.

Your brain has different sections that handle different jobs. The cerebrum, the largest part, manages thinking, memory, and voluntary movements. The cerebellum coordinates balance and smooth motions, like catching a ball or riding a bike. The brain stem controls automatic functions you don't think about, like breathing and digesting. These parts work together constantly, processing millions of signals every second.

Scientists estimate that the human brain contains roughly 86 billion nerve cells called neurons. These neurons communicate through tiny electrical and chemical signals, forming networks that let you learn new things and store memories. When you practice piano or study vocabulary, you're actually strengthening connections between neurons, which is why practice makes difficult tasks easier over time.

People also use “brain” more casually to mean intelligence or mental ability, as in calling someone “the brains of the operation” when they're the smart planner in a group project.