A bell is a simple idiophone percussion instrument. Although bells come in many forms, most are made of metal cast in the shape of a hollow cup, whose sides form a resonator which vibrates in a single tone upon being struck. The strike may be made by a "clapper" or "uvula" suspended within the bell, by a separate mallet or hammer, or—in small bells—by a small loose sphere enclosed within the body of the bell. A set of bells, hung in a circle for change ringing, is known as a ring or peal of bells. A set of 23 bells spanning at least two octaves is a carillon.