For the source text click/tap here: Beitzah 26
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A first-born animal – a behor –is considered to be holy to the Temple (see Shmot 13:12)
In the event that the animal develops a permanent blemish – a mum –then it is no longer kodesh and it can be eaten normally by kohanim.
Since under normal circumstances a behor cannot be eaten, it is not considered an animal that is ready for use on Yom Tov.
Nevertheless, the Mishnah (25b) teaches that according to Rabbi Yehudah, in the event that a behor falls into a pit, an expert can be lowered into the pit to check whether the animal has developed a mum.
If, in fact, such a mum ispresent, then the animal can be slaughtered and eaten by kohanim on YomTov.
Rabbi Shimon disagrees. He believes that unless the mum was recognized before Yom Tov began, the animal cannot be used forfood on Yom Tov.
Thus there would be no point in having an expert check the animal for a mum on Yom Tov itself so it would be forbidden to do so.
The pit referred to is a bor...or cistern...which leads us to examine the history of water cistern engineering in the ancient world an the intriguing discovery of 29 cisterns under the Har habayit.