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This document discusses 3D shapes and polyhedrons. It defines a polyhedron as a 3D shape with flat faces and straight edges, and categorizes polyhedrons into regular, irregular, convex, and concave types based on the uniformity of their faces and edges and whether their faces are indented or protruding. Regular polyhedrons have identical regular polygons as faces, while irregular polyhedrons have faces of different shapes and sizes. Convex polyhedrons have no indentations on their faces, and concave polyhedrons have at least one face with an indentation.Read less