Geometry

A circle is a round shape, with no sides or edges.

Though a circle is a flat shape, there are many other flat shapes. For example, a square, triangle, rectangle, etc. So this is not the best answer. Also, the color and the size of a shape does make determine the shape type.

A triangle is a shape with sides and angles. A triangle is the only shape with angles.

A pentagon has sides.

A hexagon has sides, a square has sides, and a triangle has sides.