When two solids are joined to make a single object their surfaces will intersect each other. The line of intersection of both the surfaces will be common and its nature will depend upon the type of solids to be joined.
When one solid penetrates the other, it is known as interpenetration of solids. Mostly it has 2 lines of intersection of surfaces, one, where one solid enters the other and the other, where the solid emerges out.
Line of intersection: It may be straight or curved, depending upon the nature of intersecting surfaces. When two solids bounded by plane surfaces such pyramid and prism penetrate each other, we obtain straight lines as their lines of intersection. The line of intersection between two curved surfaces of solids such as cylinder and cone is a curve. Also the line of intersection between a plane surface and a curved surface of two solids such as prism and cylinder is a curve.