A strong magnet helps to make high quality pictures of the brain. Next, think of the brain as a collection of nerve cells, called neurons. These neurons do all of the work we associate with the brain. For example, seeing, feeling, and thinking all depend on these neurons. To work properly, these neurons need a constant fuel and oxygen supply.
Blood is used to move fuel and oxygen to all of the brain cells.
When the brain is working on a task, a set of neurons that is associated with a particular task, work harder than the rest of the brain cells. First, the amount of blood in the area increases. Second, the magnetic properties of the surrounding blood is altered as fuel and oxygen move from the blood into the cells. For example, fMRI techniques are often used to characterize memory activities. If someone is told a phone number to dial, a specific area of the brain begins working harder to hole it in memory until the dialing task is complete.