The story is told in Tanna d'vei Eliyahu. Rabbi Yochanan ben Zakkai was walking one day when he saw a man gathering wood in the forest. He called out a greeting. No answer. He called again. Still nothing.
At last the man approached him. "Rabbi," he said quietly, "I am not a living man. I am dead."
Yochanan ben Zakkai did not flinch. "If you are dead," he asked, "what is this wood for?"
The man explained. "When I was alive, I and another man committed a certain sin together in my shop. When we were taken from the world, we were sentenced to burn each other, each by the fire of the other. So I gather wood to burn him, and he gathers wood to burn me."
The Rabbi pressed further. "How long will this punishment last?"
"Rabbi," the man answered, "when I left the world my wife was pregnant. I know she gave birth to a boy. If you would only see to it that the child is raised and taught by a proper teacher, the moment that boy is able to stand in a congregation and recite Bless the Blessed Lord, I will be lifted out of this fire. I will be free."
The teaching is heavy and clear. A child learning to answer Amen can unlock a soul in Gehinnom. What parents do not finish, children complete. And a sin shared between two men in a shop can be burning, literally, long after the shop has closed.