My favorite version yet:

A native American boy runs up to his father, the chieftain, and asks "daddy, how do we get our names?" The chieftain says, "it is tradition in our tribe to name our children according to the first thing the mother sees after she gives birth.  that is why your brother is named Prancing Deer, and your sister, we named Singing Sparrow. Why do you ask, Two Dogs Fucking?"