Where did Cain get his wife?

This is the famous question from the Scopes Trial of 1925 to disprove the Bible and it was not answered then.  Many still ask this question thinking that Christians do not know the answer.  Adam and Eve were the first two people that God created (Genesis 1:27-28), Eve was the mother of all living (Genesis 3:20), and Adam and Eve had sons and daughters (Genesis 5:4).  Looking at this evidence, Cain married his sister.  What we call incest today was not restricted until Leviticus 18:6. Also Adam, Eve, and their children had enough genetic variation that this would not be a problem (however it is a problem to marry a close relation today).  Many say that Cain got his wife from the land of Nod, but the Bible says that he went to the land of Nod and knew his wife and she bare a child (Genesis 4:16-17).  Cain’s wife was his sister, this is what the Bible tells us about this subject.