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In the context of sand casting, a core is a solid mass of material inserted into the mold to create internal cavities or geometric features that would be difficult to form with the mold alone. Cores can be made from various materials, with sand being the most common, although other materials like metal or ceramic are also used depending on the specific requirements of the casting. The use of cores allows for the production of complex internal geometries, such as passages, holes, or mixed elements, that enhance the functionality of the final cast product.