Cain and Abel - 2

And in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord. 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: But unto Cain and to his offering he had not respect. And Cain was very wroth, and his countenance fell. (Genesis 4:3-5 KJV)
By faith Abel offered to God a better sacrifice than Cain did. By faith he was approved as a righteous man, because God approved his gifts, and even though he is dead, he still speaks through his faith. (Hebrews 11:4 HCSB)

That is all the Bible wrote about the worship of Cain and Abel. God had regard for Abel and his offering, but not for Cain and his offering. The reason was not given, but Hebrews 11:4 tells us that Abel offered a better sacrifice because he offered by faith. This implies that Cain did not offer his by faith.

Many were curious to know more. Yes, Abel had more faith. But is it because Abel brought an animal sacrifice, which is what God required? That is, Abel's faith made him offer a lamb. But the Bible did not tell us that the offerings were rejected on the basis that one is animal sacrifice and the other is not. For those who want to speculate, I can offer two speculations.

Speculation 1
Abel offered animal sacrifice because he somehow knew that it is the right thing to do. Maybe he learned about fig leaves and animal skin from his parents.

Speculation 2
Animal and fruit offerings are both acceptable, but Cain was condemned for failing to bring the best, the first fruits (Deuteronomy 26:1-11). He just brought some of the fruits of the soil, reflecting a disrespectful and casual attitude. Maybe he waited till he had enough fruits, then he took the rest and offer God. You need faith to offer the first fruits. What happened if there is no more fruit after that? You would have nothing to eat.