Architects often pride themselves on the uniqueness of a design. Yet it is common for buildings that have been designed independently, nevertheless to resemble each other, visually or structurally. Usually this is because some of the ideas have been intentionally borrowed, or because shared traditions have been followed. But sometimes it is not: sometimes, the similar features have no common origin. A clear example of this is that there are ancient pyramids both in Egypt and in South America, yet, as far as we know, the cultures that built them never communicated and were not even contemporaneous. Some people whose sense of wonder has overwhelmed their critical faculties have taken this congruence of design as evidence that either there was some contact between those distant cultures after all, or there was a common cause: fanciful theories have been proposed about extraterrestrial visitors having brought the knowledge of pyramid-building to Earth.