If they only had fire, said Prometheus to himself,
they could at least warm themselves and cook their food; 
and after a while they could learn to make tools and build themselves houses.  
Without fire, they are worse off than the beasts.

Prometheus went boldly to Zeus and begged him to give fire to the people,
so that so they might have a little comfort through the long, dreary months of winter.

“I will not!” said Zeus, “Not one spark will I share with them!  For if men had fire they might become strong and wise like us, and after a while they would drive us out of our kingdom.  Besides, fire is a dangerous tool and they are too poor and ignorant to be trusted with it.  It is better that we on Mount Olympus rule the world without threat so all can be happy.”

Prometheus didn’t answer, but he had set his heart on helping mankind, and he did not give up.  As he was walking by the seashore he found a tall stalk of fennel.  He broke it off and then saw that its hollow center was filled with a dry, soft substance which would burn slowly and stay alight for a long time.  He carried the stalk with him as he began a long journey to the top of Mount Olympus.

“Mankind shall have fire, despite what Zeus has decided,” he said to himself.  And with that thought, he snuck quietly into Zeus’ domain and stole a spark from Zeus’ own lightning bolt.  Prometheus touched the end of the long reed to the spark, and the dry substance within it caught on fire and burned slowly.  Prometheus hurried back to his own land, carrying with him the precious spark hidden in the hollow center of the plant.
When he reached home, he called some of the shivering people from their caves and built a fire for them, and showed them how to warm themselves by it and use it to cook their food.  Men and women gathered round the fire and were warm and happy, and thankful to Prometheus for the wonderful gift which he had brought to them.

One chilly winter evening, Zeus gazed down from Mount Olympus and noticed fires burning cheerfully at the hearths of men and women in every village across the land.
It did not take him long to realize that Prometheus had disobeyed him and given fire to men.

from: How Prometheus Gave Fire to Men at dltk-kids.com

More poetic Prometheus HERE