There is no short answer to that question, because the brain is the most complex organ in the human body. The brain is made up of 100 billion neurons, or nerve cells, and different parts of the brain are responsible for different functions, like problem solving, movement and speech. In sum, the brain regulates practically all activity in the body. You can find answers to all of your brain questions at Neuroscience for Kids, a site which has everything you ever wanted to know about the brain, in terms you can understand.