Commentary for Avodah Zarah 47:12
אמר מר ישראל היו משמרין אותה משעה שנוצרה מנא ידעינן אמר רב כהנא כוס אדום מעבירין לפניה בשעה שעולה עליה זכר
The Master said: “It was watched by Jews from the time it was first formed.’ How did they know it [would be born a red heifer]? R. Kahana said: A red cup would be passed before [the mother] when the male is mating with her.
Daf Shevui to Avodah Zarah
Earlier we said that Jews would watch the mother of the red heifer from the time the child was formed in its womb. But how would the Jews know to watch this animal? Did they watch every cow just in case it would have a red heifer? R. Kahana offers a solution—pass a red cup (of wine?) in front of her while he was mating. This would magically create a red heifer! [Maybe if you want a redheaded kid you can try this at home].
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