Adam told his sons, “I have built a Mizbei’ach (altar). You should both offer sacrifices to HaShem, the Creator of the universe.”
Kayin had fields full of beautiful, ripe grains, vegetables, and fruits. But he didn’t want to give HaShem his best produce. Instead, he offered some simple stalks of flax.
Hevel, on the other hand, took his very best firstborn sheep and offered them on the altar.
Of course, HaShem was very pleased with Hevel’s generous offering – but less than thrilled with Kayin’s stingy sacrifice. So HaShem sent down a fire to consume Hevel’s sheep, but totally ignored Kayin’s flax.