String instruments, also called stringed instruments or chordophones, are musical instruments in which sound is produced by vibrating strings, that are actually made from animal intestine, metal wires or plastic. The strings are forced to vibrate by means of plucking with the fingers or a plectrum, striking with a light wooden hammer, or rubbing with a bow. In most string instruments, the vibrations are amplified by a hollow in the body, although some have a solid body and utilize electronic amplification. Examples of the former are violins and other bowed instruments, acoustic guitars, pianos, banjos, lutes and harps. Examples of the latter are electric guitars and electric pianos.