The hurdy-gurdy is essentially a mechanical violin. Instead of a bow, it uses a wooden wheel that the player turns with a crank. To create the friction necessary to make the strings vibrate, the edge of the wheel must be coated in resin. If the wheel is too smooth, it won't 'grip' the strings; if it is too sticky, the sound becomes harsh. The strings are also wrapped in a tiny amount of cotton wool at the point where they touch the wheel to protect the string and mellow the tone.