A pyramid is a geometric shape consisting of three sides and a base. It is also a gateway to other dimensions. It allows beings to teleport from one part of the Universe to another without lots of tedious waiting around in spaceport lounges, followed by uncomfortable journeys in low-cost spaceliners, only to find upon arrival that their luggage is lost in a black hole. Plus, pyramids are easy to build and instantly recognizable. They are the interstellar equivalent of telephone booths - all the same shape and often a little smelly.