Think of a rectangular pyramid. What does it look like? Suppose you stand the pyramid on its rectangular base and then look straight down at the pyramid along an imaginary vertical line coming at you through the vertex of the pyramid. From this point of view you are easily able to see the rectangular base.
Now leave everything as is. Borrow Mom's biggest kitchen knife, and then, carefully, cut the top (including the vertex) off the pyramid, leaving the base and part of the solid above it as they are. The shape will be a rectangular pyramid, not only the base itself but also the shape of the area formed when you removed the top of the pyramid.