A cone, a section of a sphere.
This is because the material the ballon is made of is shaped differently. The ballon will stretch and try to achieve a perfect round shape, but can not due to limitations in the way the ballon is made. One can however buy balloons that are almost perfectly round when inflated.
A cone has one vertex and one edge.
triangle
A cone has one vertex, one edge, and one base.
You can take a round cup or glass and set it in the middle of the paper and then taking a pencil you can trace the outer edge of the cup. You should have a perfect round circle upon completion.
The edge of a 3D shape is called an "edge." It is the line segment where two faces of the shape meet. In geometric terms, an edge is one of the fundamental components that define the structure of polyhedra and other solid figures.
A line?
A simple machine is the wheel, build one by cutting a perfectly round shape and fixing a rod through its centre to act as an axle. The simplest machine is a lever.
A circle?
A cone.
An edge.
A circle. A Moebus strip.