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Shape
A shape is a geometric figure that can be described with mathematics. One way to classify shapes is to describe a bigger kind of shape that the shape is one of. For example, they can be classified by their different numbers of dimensions. Thus, circles are twodimensional shapes so, like other 2D shapes, they will fit into a flat plane.
Threedimensional objects like cubes will not fit inside a plane, because they are not flat. Fourdimensional shapes made of polygons are called polychorons, and shapes made of polygons of any dimension are polytopes.
Two shapes are said to be equal, if one can be changed into the other by turning, moving, growing, shrinking, or more than one of these in combination.
Ordinary shapes
2D shapes
These are twodimensional shapes or flat plane geometry shapes. Their sides are made of straight or curved lines. They can have any number of sides. Plane figures made of lines are called polygons. Triangles and squares are examples of polygons.
3D shapes
These are threedimensional shapes. Their sides are made of flat or curved surfaces.
Special shapes
Some shapes are special. These special shapes either cannot be made in real space, or they look unusual.

