Why did God accept Abel's offering and not Cain's?

In Genesis 4, we read the following account of Cain and Abel: > So it came about in the course of time that Cain brought an offering to the Lord of the fruit of the ground. Abel, on his part also brought of the firstlings of his flock and of their fat portions. And **the Lord had regard for Abel and for his offering; but for Cain and for his offering He had no regard.** So Cain became very angry and his countenance fell. Genesis 4:3-5 The story continues that Cain continued in anger and ended up murdering Abel. So, Cain's offering was not accepted by God, while Abel's offering was. Why was this so? What was it about Cain's offering that made it unacceptable, what was it about Abel's offering that made it acceptable?