Genesis 4, Cain's Offering

Question: Why was Cain's offering not acceptable?  Answer: Since we do not have God's instruction about offerings to them, we do not know what exactly was wrong with Cain's offering. What we know for sure is that Cain's offering was not acceptable because it was not offered by faith as Abel's was. Hebrews 11:4, By faith Abel offered to God a more excellent sacrifice than Cain, through which he obtained witness that he was righteous, God testifying of his gifts; and through it he being dead still speaks. -PJ