Bells consists of two metal plates which vibrate when they are stroked along their edges with a rosined bow. When each plate vibrates at a specific tone, sand on the top of the plate forms distinctive patterns. Sand bounces on the vibrating regions of the plate and collects in those areas where there is no vibration. The patterns are determined by the shape of the plate and the spot on the edge where the bow is applied, as well as the manner in which the bow is applied.