Mar Ukva, a fourth-century Babylonian sage and exilarch, was famous for his habit of secret charity. Every day he would pass by a certain poor man's house and drop a small purse of coins under the doorpost, and every day the poor man would find the coins without ever learning who his benefactor was.
One evening, the beggar resolved to solve the mystery. He would watch. He would wait. The moment he saw the purse drop, he would chase the giver down and insist on at least thanking him.
That evening Mar Ukva happened to come with his wife. (In one version preserved by Gaster as exemplum No. 228, it is his daughter.) The beggar heard the small clink of coin on stone and burst out of the house, running after them. Mar Ukva panicked — not at being caught, but at the humiliation he was about to cause this poor man, who would be shamed by having his charity exposed in public.
In an instant, Mar Ukva and his companion turned and ran, looking for anywhere to hide. They saw only one option: a baker's oven that had just been stoked for the morning's bread. Without hesitation they jumped inside, and concealed themselves in the embers rather than let the poor man see their faces.
And here the story becomes impossible. The fire did not burn them. Not even their hair was singed. The Talmud in Ketubot 67b preserves this miracle not as magic, but as a measurement: the Holy One, blessed be He, values the dignity of a poor person so highly that He will bend physics itself rather than let a giver shame a receiver.
The rabbis concluded, from the text in Psalm 41:2, "Happy is he who considers the poor — the Lord will deliver him in the day of evil." The deliverance, Mar Ukva's story teaches, can be literal. Heaven cools the furnace of the one who refused to let a hungry man feel smaller.