Bow (music)
Horsehair is used for the hairs of the bow.  These hairs gradually fall out when the bow is used a lot.  When a lot of the hairs have fallen out the player can have the bow re-haired.  When the bow is new, or when it has just been re-haired, it will not make any sound until the hairs have been well rubbed with rosin (also called “colophony”).  The rosin makes the hair grip the string as it is drawn across.  Every violinist needs to keep a piece of rosin (called a “cake of rosin”) in his or her violin case.  They may need to rosin the bow for a short while each time they play.  There is a screw at the end of the bow for tightening the hairs.  After playing, the bow hairs should be slackened again before the bow is put away.  This is to stop the wood from warping (bending out of shape).