Genesis 4:4

"But Abel brought fat portions from some of the firstborn of his flock.  The Lord looked with favor on Abel and his offering."  Genesis 4:4

The Qbible has that "fat portions" can be translated as the "richest or choice part" and "favor" as "respect" or "regard." 

So Abel, like Cain, also brought an offering to God, but it says Abel brought the best of his flock.  It doesn't say Cain brought the worst or the leftovers, it just doesn't say he brought the best part either.  It also doesn't tell us what God thought of Cain's offering.

What we can learn from this passage is that the best offerings to God are from the best portions from our first fruits.  Those are the offerings he will respect, and to have God's respect is just an awesome thought.  Who wouldn't want that?