I was teaching the story of Isaac and Rebecca to my Valiant 10 class and said that Abraham wanted his son to marry someone of the same kinship.
"Ew!" the kids said.
"Okay, not closely related, just kinda related".
One boy asked, "wouldn't that make their kids deformed?"
I tried one more time. "Abraham needed it to be a woman of the same clan". A roomful of heads bobbed then, completely understanding that the marriage had special rules and meaning. "Yeah," one boy said, "I bet they were Tlingit*."
*Tlingit traditionally only marry someone from another moiety. If you're eagle, you marry raven. See http://en.wikipedia.org/wiki/Culture_of_the_Tlingit for more information.