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.
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.
It's called a mobius strip.