Had Cain given Abel an idea? We don't know. In any case, Abel follows suit. He turns out to be a thinker. He chooses the firstborn animals. Later, God will prescribe this in the laws of Moses: you must dedicate the firstborn animals to the Lord. Abel senses that. In firstborns you give the whole offspring. You show everything belongs to God. He also gives the best cuts of meat (the fat portions). In later legislation God also makes this a precept: to God you always give the best, nothing that is lame or thin. If you were to give that to a ruler on earth, how would he be offended! You did not have to teach Abel that you also give God, the Most High, the very best.
4 and Abel also brought of the firstborn of his flock and of their fat portions. And the LORD had regard for Abel and his offering,