An electric guitar is a guitar that uses a pickup to convert the vibration of its strings—which are typically made of metal, and which occurs when a guitarist strums, plucks or fingerpicks the strings—into electrical impulses. The vibrations of the strings are sensed by a pickup (music technology), of which the most common type is the magnetic pickup, which uses the principle of direct electromagnetic induction. The signal generated by an electric guitar is too weak to drive a loudspeaker, so it is plugged into a guitar amplifier before being sent to a loudspeaker, which makes a sound loud enough to hear. The output of an electric guitar is an electric signal, and the signal can easily be altered by electronic circuits to add "color" to the sound or change the sound. Often the signal is modified using effects such as reverb and distortion and "overdrive", with the latter being a key element of the sound of the electric guitar as it is used in blues and rock.