A bell is a directly multiplied percussion instrument. Most bells have the shape of a hollow cup that vibrates when struck in a single strong tone, and its sides form an effective resonator. The blow may be done by an internal "clapper" or uvula, an external mallet, or - in small bells - by a small, loose ball enclosed in the body of the bell (a jingle bell). Bells are usually made of bell metal (a type of bronze) for their resonant properties, but they can also be made of other hard materials. This depends on the job. Some small bells such as ornamental bells or cow bells can be made of cast or pressed metal, glass or ceramic, but large bells such as church, clock, and tower bells are usually cast from bell metal.