Revolving Sketches in OnShape
Onshape, the browser- and iOS-supporting CAD software, has a revolve feature, to turn a 2D sketch into a 3D object with radial symmetry. To create a surface using revolve, create a sketch, choose a line to use as the revolve axis1, and select shell when using the revolve feature. A surface usually isn't what you want to make using revolve, after all, it isn't even the default output of revolve! Revolving a solid, unlike a surface, requires a closed region. A red outline around the region selection field is often due to an open shape.
For simple sketches, shapes can be closed by connecting the last point and the first point of the sketch using a line. Once closed, the shape will show with gray shading on the inside:
A construction line, in the same sketch that will be revolved, can be used as the revolve axis. Even a line that is part of the shape will work as the axis.↩