In order to squeeze millions of bits onto the platter's surface, the read-write head slides back and forth across the platter's surface, very much like the tone arm of an old record player or the laser beam of a modern music CD player. Data bits are stored in concentric tracks that are logically formed on the surface of the platter when the read/write head is stationary. To jump to the next track, a small electric motor called the stepper motor is turned on briefly. The clicking sound that you hear from your hard disk when it is reading or writing files is the stepper motor turning on and off.