In geometry, the only shape that has one surface is a plane.
A sphere has only one surface.
a sphere
Circle
A hemisphere
hemisphere
A cardioid is one possibility. See link, below.
a sphere
Both. ANY 2-dimensional shape ... that is, a shape drawn in a plane, or on a piece of paper ... has only one surface.
Sphere
sphere
A 2D shape is flat and only has a surface area whereas a 3D shape is an object that has surface area and volume.
a cone.
A sphere
a sphere
A sphere is a three-dimensional shape with one surface. Or: An irregular blob, an ellipsoid, a torus, plus other shapes. The most interesting 3D object with only one side is of course the Moebus strip.
It is due to surface tension. Surface tension is only for liquids. Due to surface tension surface energy is to be minimized only reducing the area. For a given volume sphere has minimum surface area. Hence spherical shape.
A cone.
A line?