Solids are planar surfaces that have been stitched together.
1, select the lines that form a square or a rectangle. The arrow direction on each line must be head to tail.
2, execute the SOLIDS/SURFACE CREATION/PLANAR SURFACE command with the distance = 0.
3, repeat steps 1 and 2 until all the surfaces have been created.
4, select all the surfaces and execute the SOLIDS/STITCHING/STICH SURFACES TO SOLID.