A solid is a term used when talking about a three dimensional object.

By slicing through a solid, we produce a two dimensional shape called a cross-section. The figure created from a cross-section depends on the orientation or angle of the plane during the cross-section.

By taking a slice of a cube that is parallel to one of its sides, we form a cross-section that is a square which is congruent to the sides of the cube.

By taking a slice of a cube that passes through the midpoint of three edges, as if we were cutting off a corner of the cube, we form a cross section that is an equilateral triangle in shape.

Notice that a solid may form many different shapes by taking different cross-sections. In particular, knowing a cross-section of a solid isn't enough information to uniquely determine the original solid - many different solids can produce identical cross-sections.

Worked examples