Sometimes You Have to Make Sacrifices

Adam told his sons, "I have built a mizbeyach (altar). You should both offer sacrifices to HaShem, our creator."

Kayin had fields full of beautiful, ripe grains, vegetables, and fruits. But he didn't want to give HaShem his best produce. Instead, he ate everything he wanted first, and then offered the leftover crops to HaShem! Even worse, he didn't put any fruits on the altar - only common stalks of flax.

Hevel, on the other hand, took his very best firstborn sheep and offered them on the altar.

HaShem saw what each brother brought. He was 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 not Kayin's "leftover surprise."