Cain knew his wife.

We know this story is from the priestly source because there is sacrifice, there is sin, there is swift punishment.   Cain worked the soil and Able kept flocks.   It came time to bring a sacrifice to the LORD, yhwh   Cain brought "some" if his crops while Able brought the prime fat pieces of the firstborn of his livestock.  So the LORD looked with favor upon Able's sacrifice and not Cains so he was bummed out.  Yhwh said when you sin you are not accepted, you do what is right and everything is cool.  Meaning Cain's attitude was to casual, he just brought some of his crop.   So naturally Cain Killed Able.  You've heard of sibling rivalry but this is ridiculous.  These verses show God wants to be in relationship with us.  He walks with Adam and Eve and talks with them after their sin and does the same with Cain, and he wants to walk and talk with us as well.  God talks to us but we don't always listen for his voice.  Yhwh drove Cain out of the garden and now growing crops would be difficult for all humankind as punishment.  Thanks a lot Cain.   Cain went to the land of Nod and lay with his wife, or knew his wife as the case may be.  I was asked in youth group if God just made Adam and Eve and they had Cain and Able where did Cain find his wife?  Well, they  were quite an item in third grade.   Just kidding.  We have to skip ahead to Chapter 5. 1 of Genesis.  Keep in mind all the oral stores and traditions told out in the fields and around camp fires were collated into the Torah, the first five books of the Old Testament. The ancient Hebrews were not interested in details like how Cain would meet his wife and they were not interested in chronological order.  They were interested in making a point.  The point typically is sin is punished, faithfulness is rewarded.  So back to Gen 5.1, "When God (Elohim the powerful god of mercy) created man (humans) he made him in the likeness of God.  He created them male and female and blessed them and when they were created he called them man (humans). 

So that is the story of Cain and Able in a nutshell.  And I hope this has been a good lesson to you.