A cello. As the bow, made of horsehair and wood (typically), runs across the metal strings of the cello, it create vibrations. Depending on the thinness or thicknees of the string, as well as the length, it creates a differen pitch. If you place a finger on top of one of the strings, it shortens the string and creates a higher pitch. The vibrations caused by the bow on the strings reverberate throughout the wooden instrument, which is hollow and echoes the sound through the holes carved into it.
