Musical tuning
Instruments such as the piano or organ have to be tuned by people who are specialists in tuning.  For most instruments, however, the players themselves need to tune their instruments before they play.  Players of string instruments can turn the pegs at the top of their instruments to change the tension (tightness) of the string.  Players of wind instruments can make their instruments very slightly longer or shorter by pushing out or pulling in one of the joints.  Timpanists turn the taps which are around the top of their instruments to change the tension of the drum head.