פירוש על עבודה זרה 47:12

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].