As indicated above, there are as many mythologies as there are tribes and, therefore, as many sets of deities. These sets do not tend to be family pantheons as they are, for instance, in Sumer, Egypt, Greece, and Rome. Yet there are specifically understood groups of gods or spirits who are in some sense related or who work together as a unit. Examples of these are the Hopi kachina and the Navajo yeii. Often there are groups that include a distant creator or Great Spirit, an Earth Mother, and sometimes a trickster. There are occasional examples of world-parent creator deities—a Sky Father and Earth Mother who, as in other cultures such as the Egyptian and the Greek, must be separated so that creation can take place.