the barn door
Three teenagers hid themselves in a barn on the Sabbath in order to enjoy a smoke. When they were discovered, their elders decided they needed a beating for their offense.
The first youth protested. "I deserve no punishment,” he said, “for I forgot that today is the Sabbath."
The second youth also admitted to a failure of memory. "And I forgot that smoking on the Sabbath is forbidden."
The third youth raised his voice and cried out: "I, too, forgot."
“What did you forget?" he was asked.
The lad replied: "I forgot to lock the barn door."