<P> Adding even more complexity to Aztec social stratification was the calpolli . Calpolli, meaning' big house' is a group of families related by either kinship or proximity . These groups consist of both elite members of Aztec society and commoners . Elites provided commoners with arable land and nonagricultural occupations, and commoners performed services for chiefs and gave tribute . </P> <P> Tenochtitlan was the capital of the Mexican civilization of the Mexica people, founded in 1325 . The state religion of the Mexica civilization awaited the fulfillment of an ancient prophecy: the wandering tribes would find the destined site for a great city whose location would be signaled by an eagle with a snake in its beak perched atop a (Opuntia) cactus . </P> <P> The Mexica saw this vision on what was then a small swampy island in Lake Texcoco, a vision that is now immortalized in Mexico's coat of arms and on the Mexican flag . Not deterred by the unfavourable terrain, they set about building their city, using the chinampa system (misnamed as "floating gardens") for agriculture and to dry and expand the island . </P> <P> A thriving culture developed, and the Mexica civilization came to dominate other tribes around Mexico . The small natural island was perpetually enlarged as Tenochtitlan grew to become the largest and most powerful city in Mesoamerica . Commercial routes were developed that brought goods from places as far as the Gulf of Mexico, the Pacific Ocean and perhaps even the Inca Empire . </P>

Why did the aztecs built floating gardens in their capital city of tenochtitlan