Jack was an Irishman, a very mean Irishman. He was also cruel to woman and children. Jack spent most of the day drinking at the local pub and then sleeping it off during the day.
Well, it just so happened that Jack died after going on one of his binges. He went to the Gates of Heaven and the Portals of Hell. Neither God nor the devil wanted this cruel, nasty and lazy mortal. So the devil tossed a red, hot coal to Jack and told him that he would be destined to walking the night for all eternity. Jack took his pocket-knife and carved a hole in a large turnip. This was his makeshift lantern that he used to see where he was going on the l-o-n-g, dark nights.
When the Irish came to America they brought the holidays and celebrations with them. It was not easy to carve a turnip and the pumpkin was destined to become Jack's companion on the nightly walks that was his sentence.
And yes a turnip is hard to carve, see next >> Turnip Carving