In the generation after the Second Temple was destroyed, some men claimed to be descendants of the priestly lines and demanded the privileges of kohanim — including the right to eat terumah, the ritual offerings, in a state of priestly purity.
Rabbi Yochanan ben Zakkai was approached by one such claimant. The man insisted that he was of the High Priestly family and that his household observed all the required standards of purity when they ate.
Rabbi Yochanan did not argue about the lineage. Instead he asked the man a series of small, precise questions about what he did when the oil from an offering dripped onto ordinary flour — questions so detailed that they required a lifetime of training in the Temple laws of tumah and taharah to answer correctly.
The man faltered. He guessed. He gave an answer that mixed two categories the priests never confused. Rabbi Yochanan stopped him gently.
"You are not eating in priestly purity," he said. "You did not know that the oil transmits impurity through a second surface. A true kohen raised in the Temple courts would have known this as a child knows his mother's recipe. Whatever your lineage may or may not be, the life you are living does not match it."
The man went silent.
The Exempla preserves the exchange as a lesson about claims versus practice. It is one thing to say, "I am from a great family." It is another to live in the details that prove the family. The sages, especially after the Temple was gone, cared fiercely about the second — because in their age the details of priestly life had become almost the only evidence left.
(From The Exempla of the Rabbis, Moses Gaster, 1924, no. 295.)