Genesis 4:1 says, And Adam knew Eve his wife; and she conceived, and bare Cain, and said, I have gotten a man from the LORD.  This is the first recorded birth, and God is credited with giving life to the baby, Cain.  We should always acknowledge God as the source of life.  Verse two states, And she again bare his brother Abel. And Abel was a keeper of sheep, but Cain was a tiller of the ground.  Then, we have a record of the birth of Abel, and are told both Cain and Abel's occupation, so evidently some time had passed since their birth.  Abel was a shepherd and Cain was a farmer.  Verse three declares, And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the LORD.  After some time, Cain brought sacrifice to God.  This was before the sacrificial system was set up, so we must assume that this was something that Cain decided to do on his own.  This does not say that Cain brought the best of his fruit to God.  If we are going to give something to God, we need to make sure we are doing so with His guidance and that it is the best that we have.  Verse four states, And Abel, he also brought of the firstlings of his flock and of the fat thereof. And the LORD had respect unto Abel and to his offering:  Abel brought an offering to God, and we are told that it was the first and best of his flock.  We should always give the best that we have to God.  Verse five proclaims, But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell.  Some people say that Cain's offering was not accepted because it was not a blood sacrifice.  We need to remember that this was before what was to be sacrificed was spelled out and that it was a voluntary sacrifice.  I believe the reason the sacrifice of Cain was not accepted was because of his attitude.  I believe Cain made the sacrifice to bring praise to himself and not to God.  Likewise, I believe that Abel's sacrifice was accepted because his attitude was to give God the best.  We cannot give to God for any reason other than to bring glory to Him and expect Him to have to accept our sacrifice and praise us.  I believe that Cain's countenance fell because he expected praise and did not receive it.  Verse six asks, And the LORD said unto Cain, Why art thou wroth? and why is thy countenance fallen?  God asked Cain why he was upset.  Like Cain, we really have no reason to be upset if we offer something to God for the wrong reason and He rejects it.  Verse seven adds, If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. And unto thee shall be his desire, and thou shalt rule over him.  God told Cain that if his attitude toward God was right, then his sacrifice would be accepted.  God did not say that if Cain brought the right sacrifice that it would be accepted.  We should always bring the best that we have to God for His glory, and when we do our sacrifice will be accepted.  Verse eight states, And Cain talked with Abel his brother: and it came to pass, when they were in the field, that Cain rose up against Abel his brother, and slew him.  We are told that Cain talked with Abel.  I believe that it was not a polite conversation on the part of Cain.  Cain was not satisfied with talking, but was so angry he killed Abel, who had done nothing to Cain.  I believe that self righteousness was behind the murder.  We can never allow the successful worship of God by someone else to make us angry.  If we feel that someone is being more blessed, then we should simply praise God for their blessing.  Verse nine proclaims, And the LORD said unto Cain, Where is Abel thy brother? And he said, I know not: Am I my brother’s keeper?  God asked Cain a simple question that He already knew the answer to.  Cain did not answer, but responded with a question for God.  We are never going to outsmart God, no matter how we may attempt to deflect His questions.  Cain asked if he was his brother's keeper, as if he didn't know what had happened to Abel.  We can be certain that when God asks us a question that He already knows the answer, and if we are guilty of sin our only response should be to confess that sin and ask for forgiveness.  Verse ten asks, And he said, What hast thou done? the voice of thy brother’s blood crieth unto me from the ground.  God told Cain that He knew that he had killed Abel.  The truth can never be hidden from God, no matter how evasive we may be.  Verse eleven states, And now art thou cursed from the earth, which hath opened her mouth to receive thy brother’s blood from thy hand;  Then verse twelve adds, When thou tillest the ground, it shall not henceforth yield unto thee her strength; a fugitive and a vagabond shalt thou be in the earth.  Sin will never go unpunished.  Even though Christ took on the punishment of our sins Himself, they were and are still punished.