Fatty logic[1]
ויהי מקץ ימים ויבא קין מפרי האדמה מחנה ליקוק: והבל הביא גם-הוא מבכרות צאנו ומחלבהן וישע אל-הבל ואל-מנחתו: ואת-קין ואל-מנחתו לא שעה ויחר לקין מאד ויפלו פניו
And it was at the end of some days that Kayin brought from the fruit of the land as an offering to Hashem. Hevel also brought from the firstborn of his flock and from their fats, and Hashem accepted Hevel and his offering. [Yet] Hashem didn’t accept Kayin nor his offering, and this upset Kayin very much, and his face fell[2]
Our Sages note the contrast between the offerings brought by Kayin and his brother Hevel. Hevel brought from the firstborn of his flock, and the fattiest parts of them, as a slaughtered offering to Hashem. On the other hand, Kayin simply brought from the produce of his land[3]. Why didn’t he also bring an animal offering, like his brother did?