A Solid in SketchUp is a single Group or Component whose geometry meets certain conditions.
When those conditions are met, Entity Info indicates the model is a Solid Group or Solid Component.
Also notice Entity Info indicates the Volume of a selected Solid.
• The geometry must form a single* airtight vessel.
Like a perfect soap bubble … •No gaps •No holes •No leaks
• No extraneous Faces** inside or outside the vessel.
All Faces must serve to enclose the singular volume of the vessel.
• No stray Edges.
All Edges must serve to support a Face that in turn serves to enclose the airtight vessel.
Thus, each Edge supports two Faces … no more, no less.
• No nested Groups or Components.
*A Solid Group or Solid Component may contain one or more separate airtight vessels.
**Best that all Faces are oriented Front Side (white) facing out.
Understanding Solid Objects in SketchUp — Aidan Chopra
• Programming & Web Design Articles - dummies
• http://www.aidanchopra.com/web-content/chapter-6/understanding-solids
Solid Inspection/Repair Tools
TIG: SolidSolver
ThomThom: Solid Inspector²
http://extensions.sketchup.com/en/content/solid-inspector²