Carve up a rectangular prism into three rectangular pyramids. [br][br]When the prism is a cube, it's obvious that the pyramids are congruent. Why are they of equal volume when the prism is generalized?

Carve up a rectangular prism into three rectangular pyramids. [br][br]When the prism is a cube, it's obvious that the pyramids are congruent. Why are they of equal volume when the prism is generalized?