I was reading to my grandchild about Cain and Abel. The book we read said Cain was jealous of Abel because God rejected the sacrifice Cain made and accepted Abel's. She asked me why, can you tell me?
The difference in the sacrifices was really about the difference in their hearts. If you read the actual story in Genesis 4, you will read that Abel, who was a rancher, made his offering of the first of his flock and of the "fat portions" of his flock. In other words, he gave God the first and the best of what he had. Cain, a farmer, only gave God "some" of his crops. He was greedy and self-centered and apparently wanted to keep the best for himself. God rejected Cain's sacrifice because Cain rejected God's right to his heart.
Hope this helps!