Because that is how a polyhedron is defined.
Because that is how a polyhedron is defined.
Because that is how a polyhedron is defined.
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A polyhedron.
A polyhedron is a closed figure with several (planar) sides, and all its sides are planar. A non polyhedron does not have these properties, maybe it is not closed, maybe one or more of the sides is a curved surface. In some branches of topology the sides of a polyhedron cannot intersect each other.
It is so by definition.
A paraboloid.
Cylinder - a solid figure formed by two congruent parallel circles joined by a curved surface.
Sphere
cube
A semicircle
A figure with 2 circular bases and one curved surface is called a cylinder. Examples are food cans.