In SketchUp, a solid is any 3D component or group with a finite closed volume without any missing faces or faces that do not meet at an edge. With SketchUp's Solid Tools you can create new shapes by combining or cutting one solid with another.
First, make sure the geometry you want to manipulate is a solid. Select the group or component and check the Entity Info panel. If your selection is a solid, you'll see confirmation there.
To help you start creating intricate geometry and complicated shapes, SketchUp provides the following Solid Tools:
|Intersect||Remove all geometry from two solids except where they overlap.|
|Outer Shell||Combines multiple solid objects removing any unwanted interior geometry.|
|Split||Creates a solid third object representing the geometry where two other solids interact, subtracting that space from the original solids.|
|Subtract||Subtracts one solid object from another and removes it from your model.|
|Trim||Remove one solid object from another without removing it from your model.|
|Union||Combines two or more solids into a single form preserving any voids where those objects overlap.|