Suppose we have twenty balls of equal size and stack them up to make a pyramid. The "pyramid" here is actually a regular tetrahedron; its base is an equilateral triangle composed of ten balls closely ...

A 3D shape with all straight edges and flat faces is a polyhedron. Other 3D shapes with least one curved surface are not polyhedra. A 3D shape with regular polygonal faces, meeting at equal angles, is ...