The electroencephalograph, or EEG, is a machine commonly used by doctors and researchers who want to know what the brain is doing without having to open up someone's head and look inside.

In fact, even if you were to look inside the head, you still couldn't see the brain "doing" anything—the energy brains use to operate is not visible. Nevertheless, brain activity can be recorded on an EEG just by taping a few electrodes to someone's scalp. How is that?

The answer is that the brain runs on electrical energy. Individual brain cells, called neurons, regularly discharge tiny electrical impulses between themselves: this is the way they communicate. All these electrical impulses add up to create what are called "brain waves."

An EEG relies on the fact that this electrical energy can be transmitted along a thin metal wire. By placing electrodes on someone's scalp, you can get the electricity from their brain to travel across the wire and into the EEG, where it is charted on a graph. Then, by looking at a large number of graphs taken when test subjects are doing different things—such as reading, resting, listening to music, or sleeping—researchers can distinguish different characteristic brain wave patterns. Once there is a standard of healthy patterns to refer to, it then becomes possible to recognize abnormal brain waves that signal something going awry.

It isn't really reading your mind, but the EEG is a machine that can read the overall electrical state of your brain. Since its creation in 1929, the EEG has proved an invaluable tool in understanding sleep disorders, in diagnosing patients for epilepsy and brain tumors, and in a whole host of other medical areas.