We read in Hebrews
Heb 11:4 by faith a better sacrifice did Abel offer to God than Cain, through which he was testified to be righteous, God testifying of his gifts, and through it, he being dead, doth yet speak.
And we read in Genesis
Gen 4:3 So it came to pass, after certain days, that Cain brought in of the fruit of the ground, a present to Yahweh:
Gen 4:4 Abel, also, even, he, brought in of the firstlings of his sheep, and of their fat, and Yahweh approved of Abel and of his present;
What was Abel's "sacrifice?"