What is Curved Surface?
The curved surface is a rounded surface which is not flat.
Curved Surface  Flat Surface 
Real life examples of the objects with curved surface throughout are balls, globe, eggs, pipes, domes, etc
Egg 
Ball 
There are 3D shapes that consist of only flat surfaces. For example, a cube, cuboid, pyramid and prism are all 3D shapes that are made up of flat surfaces. Their surfaces are squares, rectangles, triangles and parallelograms. None of them has a curved surface. The nonexamples of 3D shapes with the curved surface are cubes, cuboids, pyramids, prisms, bricks, etc.
Pyramid 
Cube 
3D shapes such as cones and cylinders have a curved surface as well as flat surfaces.
Examples of the objects with curved as well as flat surfaces are drums, cans, cones, etc
Can 
Cone 
There are objects that can have 2 different curved surfaces. The given image shows a test tube with a curved surface (wall: cylindrical) and a curved surface for the base (spherical) too.
Fun Facts
