Once, after sacrificing an ox in an official ceremony, Prometheus did a trick. He wrapped the meat in the animalís skin and then took the animalís bones and covered them with delicious fat.
Then he went to Zeus and told him:.
"Zeus, you may choose between the two parts. But you have to take the oath that, whichever share you pick, this will take the godsí share forever. The other part will belong to the humans".
Zeus agreed and chose the bones. When he realized the fraud, it was too late, since he had already given an oath.
According to the legend, this is how the ancient Greeks started eating the meat of the sacrificial animal themselves and only offer the fat and bones to the gods.