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.