Pyramids in Prism

Carve up a rectangular prism into three rectangular pyramids. 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. 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?