When Tree Ear spied on Kang what was he making?

The idea that desirable traits can be bred into human beings and undesirable ones can be bred out is known as:

As you drive in to school, you hear a random radio-show caller arguing that sun exposure really has not been shown to cause skin cancer and that some research suggests that skin cancer is completely genetic and not related to sun exposure at all. at the time, you think that this is a silly argument and that the caller does not know what she is talking about. however, two months later, you are outside without wearing sunscreen, and you find yourself remembering that you had read something about how the sun actually does not cause skin cancer. this process is evidence that has influenced your attitude

According to the aztec history, how did the aztecs know where to build their new city of tenochtitlan? why was this a good location?