Solids, liquids and gases. In a solid like this brick, the particles are regularly arranged touching their neighbours and move only by vibrating. This explains why solids have a fixed shape.

A polygon is regular if it is uniform and its faces are all alike. The regular convex polyhedra are the five Platonic solids, which have been known since classical Greece. The ancient Greek ...

Besides the regular and semiregular solids, there are just ninety-two other convex polyhedra with regular faces. In 1966, the American mathematician Norman W. Johnson, a student of H.S.M. Coxeter at ...