Men have used limestone for hundreds of years, and one of these uses has been to make crayons. Limestone is formed by the many tiny plants and animals that live on the surface of the ocean. Among these are certain one-celled animals called Foraminifera, which have shells made of lime. When these animals die, their tiny shells sink to the ocean floor and in time they form a thick layer. This layer solidifies and is compressed into a soft limestone called ‘chalk’. Today, many crayons are also made from artificially prepared mixtures. This consists of a base of clay, to which a variety of pigments are added.